It's not that rats are more nutritious; rather, it's because ball pythons outgrow mice very quickly. At 2 feet, a ball python can easily take weaned rats which is probably a good time to make the switch. Some people prefer to start their ball pythons on rats from the get go; I have mine under 2' on mice, over 2' on rats since mice have way more calcium.

Length and girth can both be attributed to feeding. I have some July '03 females that are still under 2 feet and around 200grams, whereas Ippo, a September '03 male is now about 2.5 feet and around 500g.

At 30" long, your female is long overdue for a switch to rats. Females get larger and are heavier bodied so can really use the additional food. Not to mention, if you ever decide to breed your female, the bigger the better. Start her off on smaller rats (juveniles or so) when getting her to switch, as she may be intimidated by anything larger being only used to mice. However, once on rats you should be able to up the prey item size quite quickly. A 30" female should probably be on small to medium adult rats.