I do wonder about this. My experience seems to agree with several others, that a bigger stove - due to it's larger surface area, can keep a place warmer at a lower surface temperature. However, theory disagrees, for the following reasons:
1. Volume goes up much faster than surface area. Volume is x*y*z, whereas Area is only 2xy+2yz+2zx, or s cubed versus s squared. For example, the Jotul 600 is almost 2x the volume of a Jotul 400, but has only 22% more surface area, using rough envelope dimensions.
2. Radiated power is, meaning there is a much higher dependency on surface temperature (to the fourth power) than surface area (to the first power). So much so, that surface temperature should completely dominate, regardless of area.
I'd like to hear from a real physist on this, as I'm sure I'm missing something. Me? I'm heating a similar space to the OP with a stove the size of a 1970's TV console (only a slight exageration), and have been running surface temps of 250 - 350 F, versus a lot of people I see posting 550 - 650 F. My room is a toasty 70 - 73 F at those low surface temp's, so I do think there must be something to BAR's surface area statement.
https://www.hearth.com/talk/threads/somebody-explain-this-please.64328/