as far as i know, there are possible tweaks to the pdf creation that would help decreasing the file size.
some of them are already in use in 1.5.x, others might follow in the future.
but, as you probably guess, the easy ones have already made their into scribus... the ones remaining are the hard ones!
one issue, is that -- most of the time or always -- the glyphs are invidividually placed in the pdf. it would of course be useful to define spread of glyphs that follow a rule and place them in one shot.
but you have to detect the spreads... and be fast at that. and never have a false positive.
images and shapes are also inserted in rather dumb way.
somebody could of course add code to detect repetitions and only insert each image/vector graphic once.
and if the images have been cropped, only the visible part should be inserted in the image.
those are easy tweaks that i can understand well, even if i'm not a specialist in the field.
but getting those transformations right in every case is not a trivial task.
not an impossible one, but it will require time to get it right (most of all because the team will never accept patches that might lead to a bad pdf).
if somebody wants to give it a try, he/she is really welcome to do so!
but you probably understand that the small team of Scribus core programmers currently has other prorities (i mean: the pdf scribus creates are perfect for print... and pdfs for printing are the core business of scribus)