Smaller Aggregates not only perform and scale better, they are also biased toward transactional success, meaning that conflicts preventing a commit are rare. This makes a system more usable. Your domain will not often have true invariant constraints that force you into large-composition design situations. Therefore, it is just plain smart to limit Aggregate size. When you occasionally encounter a true consistency rule, add another few Entities, or possibly a collection, as necessary, but continue to push yourself to keep the overall size as small as possible.
