Two feet seems very thick for a foundation. That’s probably thick enough to be an airport runway.
40x20’ is also rather small as far as American houses go
Usual foundations (though it varies in different parts of the USA) would be 8 inches around the perimeter with 6 inches in the "middle" part of the floor. Of course for commercial or multi-story buildings you would see an increase in the thickness.
My house is 80x32 with a six inch poured floor and 9’ poured walls 8” thick. If i punched in my numbers correctly that’s a little over 70 cubic meters.