I, too, noticed the difference, but I think it was more due to the script and the director. I did notice, though, in US, Steven actually shouts (the scene in the kitchen after he punches Krill), and oh, my, is that a powerful voice - he could knock people over just by raising his voice, I think!
But I also think that in US, there were more dynamics between Casey and other characters. In US2, we only see him briefly with his sous-chef, even more briefly with the chefs on the train, and the scenes between Casey and his niece didn't have as much of the charm (much as I hate to say so) that was evident in US between Casey and the ship's cooks, or Casey and Jordan, for that matter, mostly because they had hardly any scenes together. Morris Chestnut was a hoot, and the scenes with Bobby and Casey worked well, but there wasn't enough of it.
Whoever wrote the script (and whoever tinkered with it afterward, as is usually the case) probably did not study the character of Casey Ryback as presented in the original film. It struck me that the character could have been named anything, but had "Casey Ryback" applied for audience draw.
Anyhow, if there was a third Under Seige, it would make perfect sense for a heavier Casey Ryback - after all, he's been a chef for a really, really long time....