You do learn lots of good things at companies, but you can also learn bad habits -- you can acquire a predilection for long meetings, acquire a taste for political point-scoring over (more difficult, less fun) compromise, and a default position of creating process over improving communication. Those things are survival skills at a big company, but very bad for small ones. The trick is to identify which of the skills you're picking up are good or bad and using them appropriately when you strike out on your own.