God there are a bunch of them.. I think it's a combo of several things:
- transition from ring to cage obviously.
- different rules, top control and clinching over valued by the judges, elbows on the ground etc
- let's be honest, sometimes they aren't fed easy match ups when they come over to the UFC (probably because they arent marketable in NA). For example Kos and AJ are nightmare match ups for Yoshida, etc.
- and let's not kid ourselves: some of them will under-perform here because of the drug testing policies.
Edit: Aoki would get mauled by almost all of the top UFC 155ers imo.