If the manufacturer decided to limit the lamp rating to 100W for a particular fitting, there was probably a good reason. Generally, the problem is the heat generated by filament lamps, in some light fittings with high wattage lamps the temperatures can get quite high. This could cause risk of fire or burns from skin contact. By the way, 125W lamps would be quite unusual, and anyway, would not be much brighter than a 100W one.