It's really quite simple. Skinny barrel means less distance between the hot bore surface and the exterior outside diameter. So heat will cross that thermal mass more quickly than a thick bull barrel. Sure it will heat up faster but also cool more quickly. The bull barrel will take longer to heat up so there will be less heat to dissipate. Bull barrel also has more surface area to dissipate heat from.
So like I said, Its really pretty simple
Cliff notes: They don't call it a varmint barrel for nothing.