Pages are exported smaller

Just to pile on, ADE is a continual frustration because (1) it's really, really bad at its job (2) it's a static product with no further development or improvements planned (3) Adobe won't remove it from the lineup and (4) there's a natural assumption that, because it's Adobe and because it's in the lineup, it's the right/best/family tool to use for EPUB viewing... an assumption one has to trip over to find out is untrue.

But FXL EPUB is almost as as bad, for many reasons. In your specific case, there is no particular reason pages would render at any given size, as FXL tries to straddle the complete fluidity/scalability of reflowable EPUB and the page-image world of PDF, and succeeds at neither. That is, it uses absurdly complex code to create an image of the layout or print page, but with scalability etc. intended to make it more flexible than PDF or raster images. At which it, too, fails.

If you want fixed pages, use PDF. If you want an e-book, use reflowable EPUB. There are no good third choices (except HTML).