Most frames end up from top to bottom, nectar or honey, pollen, and brood. The bees don't pack a cell with an egg in it full of pollen. The developing bee needs the whole cell before it emerges. The outside frames typically become capped honey with little or no brood. IF they are heavy pollen gatherers, you may find much more pollen in the brood nest also.
As far as adding another hive body, if they are covering 7 or 8 frames, I would add the box. Keep giving them space as they need it. Eventually, you'll build up a bank of drawn comb. Then you can really see them pack the honey away.