By using “Fun” is a more general sense (note the capital F)... [thinks about it] yes, they derive much less Fun from the former than from the latter per animal killed, but I don't think that one bug should count for as much as one cow, so... [thinks a little more about it] I dunno whether people derive that much less Fun from the former than from the latter per unit ‘moral value’.

Hmm. Have you by any chance considered becoming a vegetarian yourself? Because someone eating traditional vegetarian fare (or synthetic meat-substitutes like Quorn, for that matter) definitely derives more Fun per unit moral value.