I will try this question again since I got no answer to it the first time. If a designer wants to design an ecommerce site from the ground up using FC, the file size would be pretty large when handing it over to the developer, even after optimizing images. Would the developer have to "break up" the file in FB, or would the end user have to load the entire file at runtime, which would seem to get bogged down?
I don't think any optimization has been done to either the SDK or Catalyst in how it compiles the SWF files so they're larger now than they will be in the final verison. Ideally what you'd do in that case is proably us something like the Modules functionality in Flex. That lets you break your application into multiple pieces that can be loaded at different times. Catalyst won't have support for that but you can do it in Flash Builder.
In the Catalyst beta, all assets in your project are embedded into the swf, so adding assets increases the time it takes to load your swf - even if the user might not end up viewing most of those assets. Flex does allow a developer to load assets on demand, which helps to reduce the size of the swf. We are looking into supporting non-embedded assets in Catalyst, but you could always make the necessary changes in Flash Builder.