Flatpak uses it's own libaries and is sandboxed, so yeah it's normal. It's possible that your download was 1gb big. If you used a .deb package it might be 30mb's big
Edit, just checked on my distro and the flatpak is indeed 1,1GB big.
Actually, Gnome Software includes support packages in the download size figure. If those were previously installed, the size of the application is 24.6 MB ↓. You can check the actual size of the application by installing from the terminal which breaks down the packages you are downloading. On my system:
``
ID Branch Op Remote Download
org.citra_emu.citra stable i flathub < 24.6 MB
``
For me, Gnome Software also shows 24.6 MB because I already have it's required Flatpak runtime installed - it doesn't need to be downloaded again.
This package could also contain some bundled libraries which are not in the runtime which can make it larger than the .deb version.