I am having the exact same problem... These tiny banner ads I'm creating are publishing at like 75kb. I looked at my publish settings and they're the same as what I had in CS4. Yet the files are GIANT. Did you find any answers?
Oddly enough... I have found that as an interim fix I can save the file as a CS4 format as opposed to a CS5 format, then publish from the CS4 version and the file comes out about a third of the size of the file published from the CS5 version. That is stupid, but it works for now!
Still no idea.
I've just been using Flash CS4. Along with making .swf files bigger, CS5 is a lot more unstable and has the stupid AIR app for help, though I do like the improved code hinting.
I kinda wish I still had CS3 when you could put the help as a tab in the IDE - dunno why they got rid of that.
I'm seeing the same thing. I have a video Player that was 160K and now, simply by moving to CS5 - I'm now at 320K. That's a 100% increase in size!
I haven't seen much about this on the web. Has Adobe addressed this at all?