wood stove intelligence: choose the power?

[grizli09]

Hello
I am looking for a wood burning stove for my dwelling;
I begin a little to look right to left but there is something I necomprends pas.je m explains:
if I take the example 2 wood burning stove, the heater for a 12KW volume 240m3
and one just next door, 8KW for a heating volume 320m3
????????
someone could explain how it m the one with a lower power heater more volume ??
please