The size difference probably has something to do with Electron that AdGuard for Safari currently uses for the UI.
Though I have a question of my own that I have to ask... if you have a lifetime license, why are you using AdGuard for Safari instead of the (superior) AdGuard for Mac app? AdGuard for Safari is pretty limited in its capabilities compared to the full AdGuard for Mac app.
Rosetta 2 isn't something that's installed, it's already built into the macOS system. And using it with x86_64 binaries on a M1-based Mac isn't a security risk (at least not any different from using an Intel-based Mac), as again it's already built into the system. If you're worried about malware from x86_64 apps, you should be more worried about the new M1-focused malware that's out there in the wild.
If you use the AdGuard for Mac nightly version, it's a universal binary that's compiled for and runs on both Apple Silicon and Intel based Macs.