Because you love your new Studio so much and you're using it a lot more than your Mini? 😉
Only half joking there. If that's not the case, however, then that's quite a puzzling phenomenon. Even if your Mac Mini is older and thus has an older version of Bluetooth, I believe a later Bluetooth version brings better battery life, so that wouldn't explain it. You would be seeing increased battery life (at least theoretically) in that case.
I don't own a Studio but will be curious to see if others share your experience.