I was looking into tire width vs. profile recently, and found it is a complicated topic and not intuitive. Here is an interesting interactive URL about it (a wheel and tire size comparator):
Custom rims, wheel tire packages for your ride - RIMSnTIRES.com
Profile is a percentage of rim width.
My interest (theoretical - not looking at buying new rims or tires any time soon) was bigger outer tire diameter to better fill the fender arch, combined with narrow tires so the rubber is inside instead of outside the sides of the car. Narrower tires would make adding rear skirts easier (no need to bulge outwards). The other complication is offset and spacers, to assure mechanical clearances. Playing with the above link shows it is hard to have cake and eat it.
Not trying to hijack this thread -- seems related to me.