I'd never really thought about using an over-power heater before. The rule of thumb is that you should use 1w per litre, so I wouldn't have said it was ok for a 120l tank, tbh.
The only potential problem with it is obviously that it will heat itself to a higher temperature than, say, a 50w, which might cause issues for the small fish that you would fit in a 25l tank, if they got a bit too near it? Dunno. It will heat 25l quicker than a 50w, but I wouldn't have thought that would cause any issues. So long as it will fit in the tank properly, I would say it'll probably be ok.
Just one thing - a tank cycles better when it's heated to 30ish C, how are you cycling?
EDIT: IIRC, you are going to house a single betta, is that right? In which case, I can't see any issues with the temperature of the heater, that same heater would be in a 60l tank.
EDIT 2: Just realised that I had a 150w heater in a 60l tank with no issues, which is roughly the same kind of mismatch, so I say, yeah go for it.