This is a very non-technical definition, but I was told by a fitter at REI that my feet were very low-volume since they basically "rattled around" in trail runner shoes and hiking boots in the right size by measurement. In other words, my feet were far from the tops of the shoes when inside the correctly sized shoe. So I always need extra inserts in any of that type of shoe along with thick or two pairs of socks. There may be a more technical definition, but that's mine.