I believe you, but I'm very surprised. That has larger animals almost universally less efficient than smaller animals, which is the exact opposite of the expected efficiency due to Kleiber's law: https://en.wikipedia.org/wiki/Kleiber%27s_law

>a cat, having a mass 100 times that of a mouse, will have a metabolism roughly 32 times greater than that of a mouse.

And yet this is saying that a mice sized animal should be expected to have the same, or perhaps even better Feed conversion ratio than a cat sized animal.

Again, I believe you, but it makes no sense.

The larger livestock will usually have a much longer lifespan. It has to keep its flesh heated above ambient for that entire lifespan. If Mouse A lives 100x longer and puts on the same weight and achieves the same stature as Mouse B, it's spent ~100x as much energy getting to the same physiological place as Mouse B.

Again, that's simple mass efficiency. We can also talk about other efficiencies, like cost of feed versus cost of meat.