I certainly felt the soundstage get bigger when going from 8330s to 8350s (5" -> 8"). It might be psychological of course, the size difference is such that I guess a blind test would have been required. In any case the difference is not night and day, more like if I tried I could hear the separate smaller speakers as sources and I cannot do that with the bigger ones.
It might have something to do with SPL capabilities: the 8330 just seemed somehow a bit "strained" when going much over 80dB even though they shouldn't have a problem there. So, if it isn't psychological my second bet is that the bigger speakers just sound better and when trying to figure out why my brains decided it's the "soundstage".