Other reasons why they might be weaker than before (and variants):
- They ran out of strong power (having used up all of it), and now they have only weak power left.
- Most of your opponents are immune to "Mind Erasing Maimblaster" (maybe after their former boss realized such a thing and changed it).
- They were controlled by someone else using magic powers that could be channeled through them.
- They had unholy powers from the gods; due to their change, these powers have been revoked.
- They want to avoid collateral damage.
- If the power being removed is a area effect, maybe their former allies were all immune (or they had no former allies).
- Amnesia is what caused them to switch sides, so the same amnesia made them worse at their powers, too.
- It is a difference of perspective. They didn't get smaller; they just appear smaller to fit on the screen.