Currently, there is no rule dictating goalkeeping equipment beyond that he must wear a color distinguishable from his teammates. A simple rule against oversized gloves might raise scoring without the need to increase the size of the goal (another often suggested tweak to the game).
Keeper gloves are heavily padded, much larger than the size of the keeper's hands, an obvious advantage when it comes to stopping shots.
Reducing glove size is one of the less intrusive changes available...