I can tell you that the 10 gallon Grey bins from Wally-World will hold just short of 50 pounds of grain.
Divide the 50 pounds by 10 gallons and you come up with 5 pounds per gallon and 1 gallon = 231 Cubic Inches.
Divide the 5 pounds by 231 Cubic Inches and you have 0.022 pounds per cubic inch.
Monster Mill states their hopper extension is 9" X 12.5" at the bottom and 9" X 12.75". And since the taper is even you can average the two for 9" X 12.625" X 12.625" = 1434.5 Cubic Inches.
And 1434.5 X 0.022 = 31.56 pounds.
Their web site states "Adds 28 pound base grain capacity to the base hopper" and I mentioned a little earlier that the 10 gallon bins from Wally-World hold just short of 50 pounds of grain & this accounts for the error.