More water capacity isn't going to change the btu needs of your house. They remain the same depending on outside temps. Now if you were going to utiliZe 500-1000+ gallons and batch burn and only use the the stove to heat up that water once per day than you might be able to squeeze some effieincy out of the stove since there would be no idling. Other than that there's no gains in wood use Efficiency.
I would have to respectfully disagree about water capacity as far as burn times for the furnace,btus going to the house. Not many have benn able to compare furnaces side by side,but being on my third outdoor furnace I think I can put some input into this.
Our first furnace was an Empyre 450, water capacity 300 gallons,rated for 8000 square feet. My Father's house is a HUGE brick farmhouse,ours is smaller but older,needing improvements to insulation,windows etc as money and time permit. Have never calculated square footage,just went by what our Empyre dealer recommended.The furnace worked very good,just really liked it's wood.
Two wheelbarrow loads of wood about 10:00 at night to carry it through until 6:00 in the morning,so I was looking for a furnace that would last longer and use less wood.
Next was a BL 4044, water capacity 160 gallons,rated for 12000 square feet( has to be a typo,because there is no way it could handle it)!!! THREE wheelbarrow loads at 10:00 at night,stone cold and out before 6:00 in the morning,had to be out to feed it again by 5:00
Now to the present and best furnace,Heatmaster MF 20000E,500? Gallons I believe ,rated for 20000 square feet. Is it oversized? probably,but TWO wheelbarrow loads in at 8:00 at night easily does until 9:00 or 10:00 the next morning,when it was extremely cold,-36 F,it cut down to 10 hours between loads.
To recap,SAME heating requirements- furnace 1-300 gallons,TWO wheelbarrow loads for an 8 hour burn in cold but not extreme conditions,furnace 2-160 gallons,THREE wheelbarrow loads for a 6-7 hour burn,and furnace 3-500 gallons,TWO wheelbarrow loads for a 12-14 hour burn.