It probably depends on the machine but I would tend to think that it has everything to do with the mesh on which the ground up material is being dropped. This mesh is designed to allow up to a certain size to drop through and the rest goes to other places. Otherwise, I would think that you would have to retool the crusher rollers to have more or less teeth for the grinding process. WB
Any grinding mill I have used at various times (as a miner we often used grinding apparatus/various screens to make the ore bigger or smaller) can be adjusted depending on what it is being used for. Rock had to be like pea gravel so often it required changing screens to get the job done. I would think your g m has more than 1 screen right? IF the holes are wider the bigger the bypassed material will be and vice versa. Have fun.