The +90

from building a cottage on a forested grassland is the result of removing the forest. This is referred to as "chopping" by many here at civfanatics. It is a one-time bonus. Usually the number is not quite so high, so I suspect you have

-increasing modifiers such as buildings such as forges or factories, civics such as organized religion, or leader traits such as Creative, where leaders have double production speed of theatres (at least in vanilla).
I've never used the avoid growth button, and I'm not sure how many others, have, but there are other things you can do with the excess food.
First thing I would do is make sure all the buildings that give

, such as aqueduct, grocer, etc, have been built.
Then I would see if there is an advantage to continue growing the city even when lots of the food is going to be wasted on the

. Otherwise, maybe using the surplus food to run lots of specialists would work while keeping the city stagnant. (Obviously there are some specialists if you have 21 pop because there are only 20 workable tiles.)
Never underestimate the power of the whip (or the draft). Even with a few more

from the whip there will still be a higher total happiness because there will be less

from overpopulation. It will, of course, also remove a few

from overpopulation.
If there are overlapping BFCs with the city in question, you can try giving a few of the tiles to some other overlapping cities.
This article may be helpful on ways to get more health.