Must be related to hand size or something, because I find the Xbox controller considerably more comfortable than the Playstation Controller

I’m sure that’s a factor, but not the only one. I have friends with similar hand size that don’t match me and others with different hand size that do.

Ultimately they’re both very good, which is a big improvement over previous generations.

The PS1/PS2/PS3 controllers were never great for people with bigger hands, but I've noticed people that really dislike them seem to try to hold them with a full palm grip.

If you have big hands it can be a lot more comfortable to instead kind of wrap your fingers around the grips but not make contact with your palms (kind of like it's a SNES controller with some extra finger grips). I don't know if this is the intended way to do it, but it feels good to me.

And here is thought the way you describe holding them was the proper way....

It's how I've pretty much always held them...

It feels like only a monkey would full palm them...

It’s the obvious way, but still not great. The PS4 controller was a massive improvement.