If I remember correctly, actual pirated copies would have been maybe tens of megabytes, since they didn't include the CD audio. But that would still have been a hefty modem download.
The common pirated version of Quake was a 20MB pre-release leak that had oodles and oodles of minor differences to the final version. All of the actual levels themselves are playable and completable. The funniest differences are a mean trap that was removed from the final E1M1, and E2M6 still begins with the eponymous oubliette, banished from official releases until 2021 (!).
People were still replacing their 14.4k modems with 28.8s in 1996. I remember seeing the Quake 15 x 1.44 zips on local BBSes, which would've taken at least 3.5 hours over a perfect 14.4 connection so definitely a daunting phone line tie-up for the average kid (I had the CD.)
Oh, but that's why you went to the computer lab of your uni - those machines were on a 10mbit LAN and had a decent connection to the internet!
Although you still wouldn't download a CD, simply because... you wouldn't have been able to take it home with you! Hard drives were in many cases smaller than a single CD. Anything more than a few 10s of MBs would have been considered excessive.
Connecting to office LANs and going on the internet was a magical feeling back then. The speed coming from a 56k dialup was jarring. We'd have network games of Quake (rocket jumps included) and watch Real Player versions of grainy South Park early episodes at er, lunch. Good times.
More than the raw speed was the fact that the error checking, buffering, and compression on dialup added anywhere from 150-400ms to your ping RTT. You could disable a lot of it, but results could be mixed depending on the quality of your lines.
That was pretty tough in that day and age. Especially from pirate sites.
mIRC had some good servers but getting someone to keep a direct connection with you for that long was problematic too.
And then if you didn't have a dedicated phone line for the modem...