Under the helmet started out as a park thing and a lot of the big mountain dudes you see in those vids were never really heavy in the park scene. Over the the helmet is maybe more functional also cuz you can lift googles without tilting your whole shit back.
goggles fall down and get in your way under the helmet. and my helmet actually fits my head so there's no room for the goggles. also some goggles have that sticky stuff on the strap that prevents it from moving when it's over your helmet. it's just a better fit and doesn't move when over your helmet