For me, it's all about balance. My gym doesn't have separate shower stalls, so handicap status is about the toilet stalls. I won't use the handicap one if there's another one not being used, or if there's a handicapped guy close enough to the front of the line that it looks like he'd be waiting when I finished. I will, on the other hand, use it if that's the only one open and there isn't a handicapped guy close to the front of the line.
There are basically three possibilities I see in this shower situation:
1) The guy is either clueless as to why that stall is larger, or a jerk, and deserves to be scorned.
2) The guy has some non-obvious handicap, like maybe he has bad arthritis or a heart condition and wants to be near the safety bars.
3) The handicapped guy wasn't there when he got in the shower stall, so he didn't think he was causing any problem.