The limited space of Game Boy Color cartridges is more than likely the reason that many buildings and dungeons in Kanto have been scaled down or became inaccessible due to various plot reasons in Kanto. For example, Cinnabar Island erupted, Fuchsia City & Cerulean City's shared music was replaced with that of Celadon City and Viridian City, respectively, and most of Silph Co. isn't open to the public.
Some of these elements were brought back in the remakes. The music issues are solved for both the aforementioned towns. Other locations (e.g. Routes 24 and 25, Cinnabar Island, and Viridian Forest) become their own areas once again, Cerulean Cave returns, and the Seafoam Islands are changed from having a single room to a fully fledged Gym. Some buildings that were closed in the original games are now open again and offer new services: the Pewter Museum can revive fossils, and the former Safari Zone complex in Fuschia City is re-opened as the Pal Park.