What evolutional benefit is that?
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.
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.
It is theorised that it might actually have been an advantage to have smaller arms, so there may have been some selective pressure:
“What if several adult tyrannosaurs converged on a carcass? You have a bunch of massive skulls, with incredibly powerful jaws and teeth, ripping and chomping down flesh and bone right next to you. What if your friend there thinks you’re getting a little too close? They might warn you away by severing your arm”
https://phys.org/news/2022-04-rex-short-arms-lowered-frenzies.html
In case of larger species with a tremendous bite force getting bitten in the arm would probably result in very bad, and likely very short, times.
