E.g. Collect 150 food or 2500 food
Number of dwellers? Rooms? Time spent? Objectives completed?
Not too sure, but they seem to be easier and more logical with less dwellers. Upgrading 3 rooms got you a lunchbox in your first 10 minutes of the game, but as your vault grows, it demands that you perform more difficult tasks to get better rewards. It may also relate to loot. As far as I have exeprienced, loot is better when you have more dwellers. The game has no issue giving you the crap ton of boxes it does at the beggining, because the odds of you getting something insane are very low, and therefore wont be making the game too easy.
So based off rooms and population, it logically would scale and expect you to do more to get more.