Incremental

If a belt contains too much material, then the student becomes demoralised and feels to be getting nowhere.
If a belt is too easy, they do not feel to have earned the belt.

It is necessary to find a balance.
A good rule of thumb is this: each belt should each be notably harder than the previous one.
Also, the standard expected of the student should increase relative to belt.
The student should be assessed on all material learned to date, not just new material pertinent to their current belt.