For me, the larger groups become harder to manage, they are more likely to splinter off into little clicks. It's because of their attention span but also in a larger group they are less likely to feel that you are talking to them as an individual and that shortens their attention span even more.
I have been getting a lot of requests for multiple 30 minute parties lately. It's been an even split between money issues and issues with the number of kids (which I guess is actually a money issue, otherwise they would book multiple parties). I refuse to do a 30 minute party, however I don't say it that way. I have (so far) managed to talk them all into mulitiple 45 minute programs, charging full price for the first and discounting the rest. Last year I found that many of the kids had tuckered out around 45 minutes anyway, however I don't tell them that either.
And...believe it or not, I have yet to use my hula hoops.