Flatpak install: what does the "Download" column indicate *exactly*?

I would like to install pwvucontrol:

$ flatpak install flathub com.saivert.pwvucontrol
Looking for matches…
Required runtime for com.saivert.pwvucontrol/x86_64/stable (runtime/org.gnome.Platform/x86_64/46) found in remote flathub
Do you want to install it? [Y/n]: 

com.saivert.pwvucontrol permissions:
    ipc     fallback-x11     wayland     x11     dri     file access [1]

    [1] xdg-run/pipewire-0

        ID                                               Branch                 Op            Remote             Download
 1.     com.saivert.pwvucontrol.Locale                   stable                 i             flathub             < 29.6 kB (partial)
 2.     org.freedesktop.Platform.GL.default              23.08                  i             flathub            < 172.2 MB
 3.     org.freedesktop.Platform.GL.default              23.08-extra            i             flathub            < 172.2 MB
 4.     org.freedesktop.Platform.VAAPI.Intel             23.08                  i             flathub             < 13.4 MB
 5.     org.freedesktop.Platform.openh264                2.4.1                  i             flathub            < 976.5 kB
 6.     org.gnome.Platform.Locale                        46                     i             flathub            < 367.7 MB (partial)
 7.     org.gnome.Platform                               46                     i             flathub            < 346.7 MB
 8.     com.saivert.pwvucontrol                          stable                 i             flathub              < 1.5 MB

If I add up all the numbers in the download column, I arrive at approximately 1075 megabytes. Does this number mean anything?

I think that I can answer this myself now. Yes, it really is the download size.

I was doubting this because I couldn’t wrap my head around having to download over 1GB of support software in order to run a 1MB volume control, but that (apparently) is how it works.

Those are predicted / worst case values (see the <) the real downloaded values will be smaller.