Correction: When the size of one class increases, the number of students who are in another class decreases (you can also say "the size of another class decreases"), thus lowering the cost spent on instructors.

In English, when we say that the size of a class gets larger, we mean that the number of students in that class increases. So I'm not sure what you mean by "when the size of one class gets bigger, the number of students who are in one class gets smaller." This sentence is self-contradictory. Could you provide some additional details so we can better help you?