Actually, some armies DID field 24lber and over weapons, although they were mostly unicorns/howitzers or mortars. Siege cannon, as well, would be quite large.

Some ships and fortresses, using guns that didn't need to be hauled around a lot, had quite large guns as well.

However, as was mentioned, larger guns were rarer as field pieces because hauling them around was tiring and risky. If you were beaten on a rainy day, you didn't want to risk losing a huge and expensive field piece to mud or a broken wheel.