I have a pair of applications. Each is relatively form-based with 4-5 forms. They each compile to SWFs around 350-370KB. I was curious so I cut the code from one and pasted it into a copy of the other, effectively combining the two into one SWF when I compiled them. It grew by 12kb. There aren't any graphics embedded. Just a few classes, webservices, and some script.
That other 340KB, is it framework? Could it be built into the flash player and shrink our SWFs a lot?

