I feel like I should be able to figure this out for myself by doing a simple experiment...but perhaps someone else has already done the experiment!
When you shorten your stitch length you are going to use more thread because there will be more up-and-down-through-the-fabric length than with a longer stitch. How much more thread is used? Is it significant? Is it proportional to the change in stitch length? For example: if I shorten my stitch length from 8 to 12 stitches per inch will I use 50% more thread?
This is just idle curiosity on my part...