Flex, actually any other so called Rich Internet Application
(RIA) framework, takes a different approach than navigation from
page to page; that's what makes it 'rich'!
So you should start with navigator containers like tabs, view
stacks and accordion. These navigators are part of you mail MXML
and yet they would make your final SWF larger! That, by the way,
also what comes with the riches!
Now if size of MXML really becomes an issue, your next option
would be look at Modules.