The Isekai no Seikishi Monogatari BD transfer is good, but it is not a 4K remaster. It is an upscale of a digital intermediate that was likely mastered at 720p or 1080i originally. When you force that to 1080p, the file size bloats with empty data.
Unlike television broadcasts constrained by strict time slots and budget limitations, this OVA release benefits from a high frame budget. isekai no seikishi monogatari bd 720p top