Interested how volume of same tire can be different given same internal width, is this proven or just perception? Maybe one or othrr is not a true 17mm?
I for one really like 23 tire on widest rim, and have read somewhere its the peferable combo for speed
Yeah, without pics of calipers on both rims and calipers on the tire mounted to both rims at the same pressure it is really tough to say. Simply altering the pressure will change volume, due to the elasticity of the tire casing. One plausible rim difference is that the inner sidewall height of rims can vary, which will leave differing amounts of tire sidewall exposed above the beads, despite the same internal rim width. The more exposed tire sidewall there is, the more it can balloon outward.
An extreme example of this is that some of the Notubes rims are several mm shorter in sidewall height, whereas some of the hookless rim models seem to be made a bit taller than normal, perhaps to give a bit more margin for error before the tire blows off.
You are right that the narrowest tire on the widest rim is usually most aero, although aero doesn't necessarily equal "speed". From an aero perspective, a rim even wider than the tire would be best, but frame fitment and rim exposure to road damage can limit the possibilities a bit. Remember speed will also include a rolling resistance component, and in this case wider tires can reduce rolling resistance, both at the same pressure on smooth surfaces, and by lowering the pressure on rough surfaces, due to a reduction in "suspension losses". If you haven't already done so, check out Jarno's site here for some good comparisons of different widths of the same model tire: http://www.bicyclerollingresistance.com/