Another thing, the cooling effect you get from forward movement. Overheating slows you down, that forward momentum helps the sweat evaporate at a faster rate to keep you cooler. Unless it's much cooler inside, outside will usually keep you cooler- 75 degrees outside will feel cooler than 75 degrees inside. If the room is small and there are many other people running, you can also expect the humidity to be be higher insde which will make it feel even hotter.