Not too concerned. Pagecount isn't everything.
PHB: It's possible that we'll have a lot less spells than before, but those spells will be more versatile:
Instead of Fireball, Lightning Bolt, Cone of Cold, and so on, we might just have Elemental Strike, and you can choose an energy type (just like energy powers in the EPH work) and maybe even an area type (ray, bolt, burst, ball, cone....)
Add to that simpler mechanics (that will probably need a lot less detailing), and you're quite short, even after expanded classes.
So even if magic items are added to the PHB (and those magic items will probably not use as much space as they did in 3.xe), it can provide the old info on less space. It will get slimmer, but that's because of the liposuction.
DMG: No magic items in the DMG would cut a lot of space. Add to that a lot of easier mechanics, and it's easy to see it getting shorter.
MM: That's the only thing that might bother me. Unless the descriptions for monster advancement and monster abilities get shorter, and the monster descriptions follow, less pages would mean less monsters. More monsters are always welcome!