Rest assured, the Cozy Drive app only uses a small amount of space on your phone.
We do not synchronize the content of your files, but only their metadata (name, size, date, etc.). So it takes up very little space on your phone, and when you want to view a file from your phone, the app opens it from your server. If you want to be able to view certain files directly from your phone, even when you don't have a network, just click on the file and choose "Make available offline" from the menu. The file will then be copied to your phone.