Pellet fit and weight are two different things. Or, let's ask first what do you mean by fit? It usually denotes how well the pellets chamber, ie. are they easy to load or need a heavy push or two, or do they drop in without any resistance or go snugly and smoothly? A light pellet can be hard to load and vice versa.
10+ grain pellets are the default in a .177 cal D52, since lighter pellets usually fly just too fast. Within that bracket there's a range of fits. JSB Exacts usually fit the loosest, and would be one to try in a tight chamber gun. Exact Heavies are 10.34 gr. in weight.
I wouldn't shoot Crosmans out of any of my guns. With their high antimony content, they mess up the leading if you intend to shoot any other brand pellets (especially soft ones like the JSB's). The Crosmans are also low-quality, having wildly varying head diameters per tin.