For font-size check
here
(And after thinking twice, I may be wrong here at all, but removing some fontsize settings in "em" form embedded CSS-file made Analogus fine-fontsizes work at my 650, with PRS+ Zork-Edition.)
I'm also not deep in epub generation, but this 250kb filesize-thing in calibre does a internal split up, making the file "readable" for Sony PRS. So maybe there is a similar setting in "Pages".
As epubs are technicaly ZIP-archives, just open them with 7-zip or something and compare the internal structure befor and after the calibere epub-epub conversion.
Much more to find in the calibre and epub braches here at MR.
And you may want to have a look at Sigil, too. At a first glance it should be available for Mac.