Aria: great question, that I also have -- I'm worried that my e-reader is going to run out of space soon, with how big all the newer pdfs are. On the other hand, they seem to move between pages on the ereader better, so my guess is that they are using a lower compression setting or something like that, but purely a guess.
Now I'm kicking myself for not asking that on the AMA they did on Reddit a little while ago.
Maybe I'll make a new thread here, to ask, and see if anyone with connections can find out.