In theory though, shouldn't the battles be balanced around NOT having the full party bonus?

If the battles are balanced around it, then it's not a bonus, it's just...average. A bonus in this case would imply that you gain something from having a full party.

Is the "bonus" for having a full party the +10% edge the mob was supposed to have over you? It just seems kinda pointless/redundant if the encounters were designed with the bonus in mind.