If he summoned Gamabunta at the stadium, of course the audience would be extra surprised, and even scared. Second, when Naruto uses the summoning technigue, it doesn't always make Gamabunta. Summoning Gamabunta takes nearly all of Kyuubis chakra, so it's very much possible thet it mostly makes a smaller frog. And also there would be a risk... Would Naruto be able to control Gamabunta perfectly? Even Jiraiya said that he can't control it too well.