Your US passport can be completely full (i.e., have no space for a legible stamp) and you'll still be fine by DHS on return back to the US.
The US won't make a stink about your full US passport. Enjoy your trip home without any worries related to this.
If you have time while overseas (e.g., HKG), I'd get more pages inserted at the US consulate/embassy (just to avoid having to do so in the US).