This seems plausible to me from a perspective of "these animals likely are less capable of suffering", but I think you're missing two things in your analysis: ...(1) the degree of suffering required to create the food, which varies between species, and ...(2) the amount of food provided by each animal.

Additionally, when there is a burden of evidence to suggest that nutrient-equivalent food sources can be produced in a more energy-efficient manner and with no direct suffering to animals (indirect suffering being, for example, the unavoidable death of insects in crop harvesting), I believe it is a rational choice to move towards those methods.