its not about water capacity at all. lets remember that, as today's most efficient stoves hold the fewest gallons of water. mine is currently holding 60 gallons, and some only hold 30.
its about heat transfer, not heat storage. if your furnace isnt able to transfer enough btu into the water, as fast as your pulling it off, its gonna stay behind regardless of how many gallons you have.
but your problems definetly sound like something id love to hear more about, either the stove is just flat out over rated, or maybe there is design issues within the install. thats very possible
Even though all manufacturers show max btu ratings and say they can heat x number of square feet, its all flat out bull.. first of all, max ratings doesnt matter, thats with the draft completely wide open in the lab with perfect wood, and maxing sure the firebox is completely stuffed.... what i would like to see more companies do is give you a rating of how many btu can this stove provide over a give time period, lets say 8 12 14 hours or something like that. Then as a dealer when you show up for the job, you can do a heat loss caluclation on the home based on size, number and quality of widows, several different factors to determine the size furnace that would work best for them.
I've saw 6000 square foot homes that heat far easier than a 1500 foot farmhouse.
but lets definitely hear more about this situation........ feel free to ask questions here or even call.. this is a chance to learn something for sure