I think you're right that micro frameworks haven't permeated into the PHP community very much. I don't personally see their point, for a couple reasons:
First, the major PHP frameworks are autoloaded. So no matter how many classes or files are included in a framework, only the parts you actually use are loaded.
And second, if you're building a large-scale app, then you'll want all the features and structure that a framework provides. And if you're building a small-scale app, then a few extra milliseconds won't matter anyway.