Well, they sold out ~5000 tickets in 12 hours. True, that's not small, but clearly there's more demand than they are providing for and a lot of developers got left out. It's the same size as Google I/O for comparison.
Splitting off OS X from iOS would probably help a bit but Apple won't do it.
The last couple of years, Steve has said that he's sorry he can't find a bigger venue. Perhaps that's the problem? Which venue could the conference use to support more attendees?