Google play services is a monolith, that does a plethora of stuff on the phone, including features like quick share, location services, various Firebase APIs for instant notifications stuff, find my device and whatnot, so I think the size is pretty reasonable.
It makes sense if your phone is well supported in the custom ROM comunnity and doesn’t receive feature or security updates from the manufacturer anymore. I am already 3 Android versions past the last supported Android version by the manufacturer thanks to community maintained ROMs. (Xiaomi Poco X3 NFC).