The difference in width is that 700 has three letters and 29 has two Really there is no set threshold at which one calls narrower rims one term and wider the other term. (and don't forget about the hybrid bikes)

While there are guide lines for rim width and tire width best fits people will mount whatever they find works. As the tire approaches the rim width the ride gets more and more firm as the side wall gets more vertical. as the tire gets wider and the side wall bulges out beyond the rim the ride gets more comfy but less reactive in changing directions. At some point that is rim/tire specific (and unique to a degree with each combo) the tire won't tend to seat well enough in the rim to allow either the pressure levels wanted or the cornering forces. Then the tire will pop off the rim (burp). But the same model of rim or tire might see this differently then another identical combo. Some rim manufactures have charts for the tire width range they suggest for a rim. Just as some tire manufactures have pressure level charts for various tires and their widths. Andy.