Is my ball python under weight or just a slow grower?

[Bogertophis]

...They're more nutritious compared to mice

That's not true- according to studies I've read rats & mice are quite comparable in terms of nutrition, but statements like yours are often repeated.
(There are minor differences, of course, but not enough to justify choosing one over the other on the basis of "nutrition".)

And keep in mind that BPs didn't evolve in nature eating either one- not domestic rats OR domestic mice.