TL;DR different modes of thermal transfer can transfer heat at different rates. There are a couple factors at play that help it absorb heat faster: 1) Convecti

TL;DR different modes of thermal transfer can transfer heat at different rates. There are a couple factors at play that help it absorb heat faster: 1) Convection rather than radiation. As the hot gasses travel through they stove they are in contact with the mass and can transfer heat more directly to it, which then spreads through the mass via conduction. This is aided by... 2) Newton's Law of Heating and Cooling, which basically says "the greater the temperature difference, the faster the heat transfer". Since the gasses from the combustion are around 1000 degrees or higher (at least initially) they transfer heat much faster to the mass (several hundred degrees delta) than the mass transfers it to the room temperature air (one or two hundred degrees delta). Similarly, much of the radiation of the hot inner mass is absorbed by other parts of the hot inner mass; only the parts near the outside actually radiate heat away from the mass.