In this case, there is no size jump, from level 10 to level 15 it takes the same amount of space, and the amount of people vary a bit. I did originally check the wiki page but it looked like it only jumps a bit , not the big percentage I saw when upgrading. I think the wiki shows the total time from the original amount of time. I don't remember the last leverl which had the same footprint space wise, jumping that much.
Maybe I am missing something