Outdoors is outdoors; temperature will be the same regardless of whether there's a roof. Also, cooking outside invites crippling all of your colonists with food poisoning because the outside is inherently dirty. To avoid the problem of the campfire overheating the room, I suppose you could remove the roof of the kitchen. No roof means you can't control the temperature, but as long as you have four walls and a floored tiles, you can at least make the room clean. I've never tested that, however, because I hate playing in biomes without wood or food. And as long as you have wood, you can build passive coolers.
I don't understand how any of this results in the slice being larger than the loaf.