'Perfect fit' fit.

I bought Griffin one of their harnesses after such excellent recommendations on here and I followed the measurement instructions on the website and the whole thing does seem to fit. However the nylon webbing on the bottom of the girth strap is sewn right up against the edge of the fleece and is just wide enough to rub against his arm pits which I think is defeating the point of the fleece really, do people think that if I changed the XL piece I have for a L then the webbing piece would be smaller and fit against his chest as opposed to bridging the gap between his armpits? It must be quite uncomfortable for him as well as potentially rubbing the fur there away and since I think he'd be wearing it quite a lot whist we're away next week I want it be comfy for him!! Any advice peeps?!!