If you brood in ambient air temperatures of 72F, it just doesn't take that much to raise the temperature needed for the chicks. Frankly, a 60 watt bulb can do this. There are many variables. How far away is the bulb situated from the surface? Raising the bulb serves to lower the temperature, something that is needed with each passing week. A 125watt bulb has more power and a 250watt, red brooder bulb has incredible power and is also a fire hazard. Secure the heat lamp with a secondary wire support for backup protection. Brooding in a cold shed or garage requires more power as the surrounding ambient air may be cold.
The idea of brooding isn't to heat the entire brooder to 90F. The brooder heat appliance merely mimics the body of the broody hen. She only heats an area of approximately 1 square foot. She cannot and doesn't heat the entire coop. The chicks under her come and go. They'll explore in 45F temperatures. When the need warmth? They scoot back under her wings.
The brooder only needs to provide a heat spot, roughly the size of the hen's body for each 6-8 chicks. We don't want to heat the entire brooder box to a high temp. The chicks need cool places to go and refresh themselves. Hope all this helps.