I am playing an OCC game on small for domination with 7 opponents. At the start of the game, it requires 37% of world population. As the game went along, it went up to 62%! This actually became a bottleneck to a fast win as opposed to land %.

What factors affect the population requirement changing over time? I killed 5 opponents. Does that matter? Do vassal(s) matter?

The pop score needed for domination victory is defined as the Percentage of the worlds population held by the SECOND most populous player +25%
if you're killing off people, are the other AI's resettling their land? if so they will increase their percentages and make your life harder

The requirement is 25% more population from second best. Colonies change the land percentage (IIRC) since the number of teams is changed, normal vassals don't change anything.

EDIT : I see that NM types faster. :)

Thank you! That's an awesome simple formula, consistent with what happened in the game.