Mama broody does not heat the entire area. There is only a warm spot under her. The chicks explore, then dart under Mama to get warmed up, or if she or they sense danger. They do fine out and about exploring, even when it's pretty cold out. So, IMO, a large brooder is much preferable compared to a small one. It's great if the far end of the brooder gets down to the same temp as the surrounding area. Are you brooding them outside, in the house, or in a shed/coop? The only other possible issue might be if they get away from the heat lamp and huddle there instead of going back to the heat source. Sometimes, they're a bit um... not so smart in the first few days. That heat lamp does not know how to cluck them back to safety the way Mama broody does. Hope you have it covered. They grow those wing feathers mighty fast, and will be flitting around like parakeets by the time they are 2 weeks old!