Is it realistic that a minecraft server is 700GB+ big?

It's realistic but not likely. A player on an End raid hunting only for elytra can generate 250MB of data per hour, and afking in a flying machine may produce some 5GB per day, but 250GB for 20-30 players in a month would normally either require organized effort to bloat the save, or a malicious user mass-producing save-bloating items (like copies of written book filled with random UTF-16 characters, stored in a way that prevents good compression). The most plausible reason is some buggy plugin/mod/datapack generating obscene amount of logs or other data.

To wit, Hermitcraft 6, an extremely well developed server of ~18 players after a year and a half of playing - but with chunks the players either never visited (just saw from afar) or spent less than 2 seconds in (flew over on Elytra) removed, is 2.4GB. 2b2t, the oldest anarchy server, after nine years and and over 450,000 players visiting at least once, was 7.4 TB.