I suspect it's just that people overestimate the weight of mice. If you put them on a scale against rat pups, I think the pups are actually heavier--so you're actually feeding the BP more when you feed it a rat that looks the same size as an adult mouse.

In terms of nutrition, the adult mice have fully calcified bones, so they are probably more nutritious--it just takes more of them to make a meal!

I prefer rats. Much easier. And mice are nasty, quick, and a pain in the ...