UHD Blu-ray doesn't introduce new disc formats, but compatibility with the variant does require support for 3- and 4-layer discs (2-layer, 50GB, discs are also still supported for UHD). These aren't new for UHD, but they were introduced after the regular 1080p standards were set--nothing really stops a 1080p movie from using them too, except compatibility with very old players.
Still, as other commenters note, these large disc sizes accommodate for high quality. If we have a 100GB disc format, and you want the 4K video to appear in the best possible quality, you will choose some encoding options that fill up most of that 100GB disc, minus whatever the special features and audio tracks cost. Most consumers won't care about the size: it's on a disc, they're not ripping it to a hard drive of their own.
Still, as other commenters note, these large disc sizes accommodate for high quality. If we have a 100GB disc format, and you want the 4K video to appear in the best possible quality, you will choose some encoding options that fill up most of that 100GB disc, minus whatever the special features and audio tracks cost. Most consumers won't care about the size: it's on a disc, they're not ripping it to a hard drive of their own.