What to put in 60A GFCI breakers in Subpanel?
The problem is the breaker needs to protect both the loads and the wires. There are (at least) 3 different ways your 25A heater could fail, requiring a breaker trip:


Short Circuit - That is easy. A short pulling hundreds of Amps (or whatever the service will supply) will trip a 60A breaker almost as fast as it would trip a 30A breaker. Good
Ground Fault - A ground fault will trip a GFCI breaker at very low current, no matter what the rating of the breaker is (30A vs. 60A). Good
Moderate overload - This is the real problem. If you have some sort of fault in the heater - perhaps a component starting to fail but not totally failing (because if it totally failed it would either fail open - safe - or fail closed - short circuit). BAD In this situation, you could have 40A or 50A flowing through and it would not trip the 60A breaker. But it would melt your wires and start a fire.