Doesn't it require offline map files of some sort? These can be huge for some places.
I have easily 15 GB of map files on my phone (not counting POI lists) and that only covers Germany and some of its direct neighbours.
What app? Some seem to have better compression. OSMAND (which is probably the closest they would use) for western switzerland, french rhones alpes, franche comte, alsace, aosta and BW, including wikipedia, terrain, contour and standard maps is under 2GB for me. The equivalent on mapy is 4, and on offlinemaps it was closer to 10 iirc but I deleted it
But keep in mind cycling maps are often just roads and a couple POIs if you're lucky, they don't have all this detail so as long as you don't want an entire continent or manage them before travel it should be fine.
https://www.userbeam.de/ Check the file sizes