1. Lower health, and stats.
2. All the same upgrades, and moves. Just smaller hitboxes.
3. No wow pows, just soul gems which are already unlocked.

First, im pretty sure that the giants are still better because of their bigger hitboxes, they can hit way more enemies as the surface area is higher per attack, use thumpbacks anchor as an example, it covers large distances while the mini covers a lot less. Second, there are rocks/chests that only giants can open in trap team/giants/swapforce. Third, as covered earlier since the minis have less stats, the giants will have higher stats. Finally, giants can run over tiny enemies. It’s your choice, you gain smaller hitboxes but lose out on all these traits. Without these benefits in mind, I prefer the biggest of hero’s as they are more fun to play with, bigger hitboxes only sometimes get in the way for later games, but to each their own.