I’m not sure of the answer, but generally not everything has to have an evolutionary benefit. As long as it isn’t detrimental to a species reproducing, it will continue to exist in the population.

Someone will likely chime in with a more complete answer but the short answer is large therapods had big heads because they needed big strong heads for killing big strong prey.

If a stronger bite results in more successful kills, that creates a selective pressure towards the individuals with stronger bites. Weak bite dinosaurs die. Strong bite dinosaurs live.

To get a stronger bite, you need a lot of features. (Ex. more muscles, different structural elements) and this generally leads to heads getting bigger because big heads have more places for muscles and will then bite harder. Plus, bigger heads can bite bigger things (like, bigger necks).

The opposite is true for the tiny arms. The arms are not tiny because it's beneficial for them to be that small. They're tiny because there's no reason for them to be big.

If you have two individuals. They both have big powerful legs, and big powerful heads but one also has thick ole arms. If those arms don't provide any advantage to the individual, then they just cost energy, and put that individual at a disadvantage because they spent energy on arms while the other guy did just as well without them.

There is a much better, much more science-y answer to this too but I hope this helps in a more basic sense.

And I bet there are some cool YouTube videos and such on this exact thing if you want to do further research.

Keeps them from masturbating, allows them to transmute their sexual energy into raw power when not actively engaging in sex.

There was an evolutionary trend among carnivorous theropods that the larger they grew the less they used their arms to grab the prey: If you can either inflict a deadly bite or grab the whole neck in your jaw, grabbing with claws becomes unnecessary. Two lineages took it particularly far: in abelisaurids, such as carnitaurus, the arms became vestigial and in some cases completely disappeared.

In a separate theropod lineage, T-Rexes, while much bigger than any abelisaurid and with a bigger head, did not have vestigial arms. Their arms, tho tiny, still had bones that locked into each-other and seemed to have muscle attachments. There's been several theories as to why: This certainly meant that T-Rexes still used their arms, but how? The main theory is that juvenile T-Rexes had a different morphology as their adult counterpart. They already hunted, but much smaller preys, and with a different technique, so they would've still used their arms. On top of that, there's the idea that adult T-Rexes might've slept on their bellies and their arms would've been useful to bush themselves back up.

Another reason for T-Rex's large heads, in addition to their big jaws being terribly efficient, is that it may help with placing their center of gravity over the knees, which is useful to keep balance while walking or running.

They probably didn't need their arms for how they hunted. Same as modern birds. Think of how an eagle kills and then eats its prey.

There were some articles last year that said they think T-Rexes had small arms so they don't get bitten off when competing for food with other T-Rexes.

Sauropods had tiny heads and huge legs. So it varies. Evolution is an amazing thing.

Ah man I thought this was on dad jokes for a minute.

arms wont just "go away" when an organism evolves to use them less, and eventually would become vestigial. However I think that this question relies on a sweeping generalization that is not backed by evidence, only supported by media and the way we usually see dinos depicted

Not an expert here, but I think you’re looking at this particular example backwards. Evolution isn’t a “smart” process that picks and chooses favorable traits, it’s more simply “this animal with these traits was able to survive and mate, so these traits get passed on”

So start with what was before the dinosaurs, to my recollection that would be some kind of amphibian, living in both water and land. Probably has four equal-length limbs and walks on all fours.

Whatever exactly happened between A and B idk, but eventually you get to the dinosaurs that walked on two legs. And that there is the answer. They used their legs more, their arms became less important to their survival techniques, and so over time they shrunk.

Survival for those guys was “run fast + big jaws and sharp teeth”. Long tails for balance while running, not much use for the arms. They didn’t shrink because it was an evolutionary advantage, they shrunk because they weren’t doing the heavy lifting. Terrible pun intended.

Which dinosaurs? Predators usually had relatively large heads because big head > big jaw > kill better and bite off more meat. But herbivores usually did not, as they could just focus on plants (instead they could develop longer necks to reach them in various places); some species such as Stegosaurus had rather famously tiny heads.

Tiny arms are associated with Tyrannosaurus and similar large theropods, but lots of other dinosaurs had relatively large arms, such as Dromaeosauridae ("raptors"). Their arm size probably reflected how much they were used during hunting - raptors' much more so than T-rex's, the latter probably relied on its jaws primarily. Of course discussing "arms" of various gigantic four-legged sauropods is pointless...

Basically there's too much variety among dinosaurs to answer your question directly.

So just because a living thing has a certain feature doesn't mean that feature is advantageous. It simply means it doesn't hinder reproduction. Dinosaurs in particular are also difficult to study since we really only have bones to go off of. Animals alive today often look nothing like their bone structure suggests so we might be thinking of dinosaurs all wrong.

From some googling, it seems scientists are guessing as the heads became larger, the arms became smaller. Their giant heads are thought to provide bone crushing jaw strength. Maybe their giant heads used up so much energy that maintaining complex limbs was too much. Another thought is that they ate in packs and limbs would get in the way while pack eating.

Arms cost calories. If you don't need it, better to have less investment in it

The best answer comes down to two factors; oxygen and food. Oxygen levels were much higher 45 million years ago than they are today, and exposure to high amounts of oxygen have shown to drastically increase the productivity of the human body, so why not the same for animals. Second comes down to food. The more food, the more of an animal. The bigger the prey, the bigger the preditor needs to be or they need to be in greater numbers. When creatures like brachiosaure exist, it would understandable be difficult to kill it when small, thus animals that were larger would have better chances, or conversely animals that worked together. Of course, this is somewhat speculation and there could be other factors involved, but this is the simplest answer that makes sense.

I'm sure someone else can chime in with a better reply.

First, not all dinosaurs have large heads and tiny arms. But you are right that some were like that, like famous Trex for example.

The large head is so that these dinosaurs can have a large mouth. A large mouth means they can attack larger prey with it. That's beneficial if you are big, because you can eat with one kill as opposed to spending the whole day trying to find enough small prey. It's also beneficial if most prey available is large.

The small arms are simply because they didn't need them to hunt or defend themselves, or to move around. The mouth and legs alone were enough as these dinosaurs evolved, and so the advantage of having small arms is that you don't waste body resources and energy in growing and maintaining unecessary limbs.

arms were evolving to become vestigial

Start with long arms and walk on all fours. Evolve to walk bipedal and have shorter arms. The dinosaurs you're thinking of are just in the process of evolving and might have eventually evolved to have no arms unless those little shorties are advantage somehow. (Holding on during mating, evolving towards wing development) Some modern snakes still have little nubbins where their legs used to be.