I don't think the BotCon venues could comfortably hold more people, as they are crowded enough already (especially the dealer room and presentation room).
Just remember, they market their convention to major fansites and once they sell out of the (little more than) 1000 box-sets/full packages, we don't hear much from them.
They do some local TV commercials and radio advertising, but each year when I talk to comic stores and toystores nearby (in the days beforehand), they never know anything about BotCon. You'd think there'd be some targeted advertising at comic shops or even stores that sell toys, so that they can stock up or even have flyers near their TFs toys.

I've never seen on TV during the week of BotCon anything about it - either advertised or in the news... and the only time I ever saw first-hand some advertising, was in 2009 when *HASBRO* were promoting BotCon at the nearby shopping district.

Once FunPub sell their boxset packages (which ends up being about a 1000 people, when you take into account non-attendees and people buying up extras), they don't seem too fussed about how big or small the walk-in crowd is... as if it is just a bonus.
If they sold their full packages with the boxset being an add-on, they could attract more people who may not want the toys or afford them - and it wouldn't look like they are "sold out" of full packages once the boxsets sell out. There is almost a stigma to registering after the boxsets sell out, as if it isn't worth it.

Sorry... getting a bit off topic.