HDRi images aren’t about resolution – they can be saved lo-res or hi-res. They’re unique in that they store ‘high dynamic ranges’, namely various exposure levels of light. JPEG’s and other standard image formats store normalized pixel information on a 0-1 scale. HDRi’s store pixel information above 1, which is *closer* to capturing reality’s lighting levels which are in no way limited to a 0-1 scale.
See wikipedia for more info: https://en.wikipedia.org/wiki/High-dynamic-range_imaging
