I would not say most of them, though that's true for some (as both given here, Agricultural and Rich)
All industrial worlds are High Pop too, and can be Poor, Vacuum, Asteroid, Water, Fluid, and many others
I'd say mostly from services, as most high density population zones are today in Real World...
Also, I guess most High Pop worlds have a good degree of industrialization, even if not considered Industrial as per trade code, and are probably self suficient in food production , though probably agricultural products are in high demand for variety and as luxury products. Hydroponics and recicling would be, again I guess, enough as to avoid mass starvation of the population, but variety and quality of food may suffer.