The overall rule is time and quality.
We have a certain amount of time that we can make the show. Categories with more entries tend to have more "high end" submissions so we try to accommodate for that, however, we do try to have at least 5 videos per category, then we add on more in that category while watching the overall length of the show. This can change if say some of the higher scoring entries are much longer. In that case we may only add 1 or two extra beyond the 5 minimum, versus having enough time to add a higher number of shorter entries.
I hope this sheds some light on why we tend to have longer run times for Action and Drama. These are the most popular submissions. Sometimes we put EVERYTHING we get into Comedy or Experimental and still barely hit the 5 entry mark. I know that Comedy is the most difficult to do, but if done right, it's one of the main things that the audience tells us they're looking forward to. Ironically, we tend to get the fewest entries for that each year. The audience always say they want more Comedy. Perhaps by creating awareness in the AMV community, we can give them what they want, right? I hope so. This is why we have been trying to show everyone some of what goes into making the show.