Using these instructions I successfully setup the ProtonVPN app on a standalone Fedora minimal template and the disk usage reported by Qubes is about half the size of a similar one based on a Fedora XFCE template.
Disk Usage:
fedora-40-xfce ~9000MiB
fedora-40-minimal ~4600MiB
This is still monstrous in size compared to a WireGuard AppVM (between 100~200 MiB) but with the official app you get access to the ever-changing list of Proton VPN servers.