I agree the right light or something around it is causing the issue. The hot wire you noted is getting hot because of a current (amperage) draw that is higher than it can handle. Since it's around the right light, it adds to the probability that it is the culprit.
As Rubi 4 noted, if there was a direct short to ground, then the 25A would blow right away too. That seems to indicate that something (probably the light) is just drawing too much current.
One thing you can try to isolate the problem to the light is to disconnect it and then see if everything works ok with the 20A fuze. If it is, that doesn't absolutely mean the problem is the light, but given everything you've described, that would be a pretty good guess.