There are two ways to store map tile data like OSM. One is as pre-rendered raster tiles (256x256 png images) and the other is as tiles of vector data that get rendered either on the fly by dedicate rendering middleware or rendered client side by the browser. Vector tiles are anywhere between 20-50% smaller that corresponding raster tiles. Raster tiles was the standard way of doing web maps up until a few years ago, when client side rendering in the browser became feasible and now most people use vector tiles.