Most cats are self-regulating when it comes to food. That means you can leave dry food out for them but defitely feed them canned!!! and raw/frozen too! and when you eat fish or beef, buy extra for the kitty. :) Wellness, felidae, are good brands. do NOT buy pet food from regular grocery stores, it's from road kill and the chicken parts that don't make it into soup, like feet and heads. Seriously.
Butt pee is just that - toxins/bile that is liquid, lacking fiber from the diet.
I think it's a fair assumption that the larger stones are older.