The secret here is insulation and removing air flow. With 100% perfect insulation(impossible, but we can get closeish), any amount of heat generation will eventually heat the room, smaller room heats faster. With no air flow, the air cant efficiently transfer heat and you dont even need insulation. As long as you generate more heat than the air can transfer, the room heats up. That one doesnt really work that way though, because hot air generates it own air currents which then raises the thermal transfer of the air. But the idea still stands, you just... do actually require insulation in the real world.