How come whenever you encode at 23.976fps the file size is smaller than if you encode at 29.97fps?

Because there´s 6 frames less per second

For every second of video there are 24 frames encoded @23.976 and 30 @29.97. That means there are 1.25 times the data that must be stored when using 29.97.
Therefore you should expect your files to be about 1.25 times the size when you use 29.97.

Regards, Tenra