TIG punches heat into the material slower, generally. TIG is slower in general. With that, you have a larger amount of total heat in the TIG'ed part than MIG. The surface area is actually about the same; the entire part. Soon as you start putting heat into the surface, it starts transmitting it out to the whole part as well as radiating it off the part. With MIG, you are "in and out" much faster, so the rest of the part is generally not heated up much by the time you are done. Your weld area cools faster. That is why you can't TIG sheetmetal for a body. Too much heat, too much expansion, too much warp.