You can find the folder /var/lib/flatpak/repo/objects
is big/large maybe 5GB+

$ du -h --max-depth=0 /var/lib/flatpak/repo/objects

List installed flatpaks and its sizes, dependencies (runtime) and parent packages (ref):

$ flatpak list --columns size,version,name,app,description,ref,runtime

Removing some app flatpak also remove its dependencies (runtime), example:

$ flatpak remove com.pypyrev.linuxdcpp/x86_64/stable
Code:
       ID                                           Branch          Op 
 1. [-] com.pypyrev.linuxdcpp                        stable          r 
 2. [-] com.pypyrev.linuxdcpp.Locale                 stable          r 
 3. [-] org.freedesktop.Platform.GL.default          20.08           r 
 4. [-] org.freedesktop.Platform.Locale              20.08           r 
 5. [-] org.freedesktop.Platform                     20.08           r
Any feedback to this?