Their punching power would be diminished. Or to explain better, not their power output, but the effect their punches have on an opponent.
You're talking about pressure, but it's not that important in boxing. Punching area is small anyway, the effect wouldn't be noticeable. Meanwhile, bigger gloves make it easier to transfer the energy to an opponent without landing it perfectly. You play with much bigger margins of errors right now.
a bit but in real fight the opponent has same gloves so each per punch might damage less & this mean in long fight it can work out more strength after few rounds because taking a bit less damage too. & Factor of hands : less damage accumulate here also . it even it up