OSM maps are vector based - but not all apps use vector mapping and convert them to raster tiles which require much more memory space.
For example OSM vector maps for England south of Yorkshire occupy 1.1 GB, OSM raster map for Hampshire alone is 0.52 GB.
OS & Viewranger use raster maps, OsmAnd, for example, uses vector maps, as do Garmin GPS.
Vector maps, apart from useing less memory, zoom smoothly with no jaggies.