Belt width also has a pronounced effect, a wider belt shifts the whole range "upward" (less acceleration from stop and a higher top speed)
So you'd better change one dimension at a time if you want to learn anything because a longer and narrower belt might give the same performance as a shorter & wider one (or the other way around, I'm too tired now to really think about which way it works)
When I fitted a new belt I had to put a few 0.5mm washers against the drive boss to achieve the same combination of acceleration and top speed as before (i've got plenty of top speed for the place where I live, and I like some extra kick "off the line").
As the belt wears, I can remove one washer at a time to keep the result within the same range...