Others can answer better than I, but a first priority is to be the vassal with holdings closest to the newly-acquired fief. If you're out, conquering places closer to other lords' holdings, they will be favored.
You pretty much get zero recognition for being the rainmaker of the kingdom. And there doesn't seem to be any weighting (as far as I can tell) regarding how many fiefs you already own. What I mean is - you're not in a socialist country and rewards won't be distributed equally.
Once I learned this, I began a new run-through and targeted fiefs based on their nearness to my own holdings. I got 6 cities plus a handful of castles before I told Unqid, "Good-bye. I'm off to start my own kingdom and then come back and take these places again."
PS - I made the Aseris too powerful before I left them. They're holding off the Khusaits and whomping everyone else.