When all other things are equal (temp, oxygen content of water, etc.) the fish with the smallest mass will produce the least waste. There are other things to consider, such as the fish's lifestyle -- a zebra danio (much more active) will produce more waste than a similarly sized cory catfish (that spends a significant amount of time sitting on the bottom). But, for the quick, big picture: fish mass -- which would be close to fish volume -- is the best guess.