Quick update. I just tested this out by looking at the data usage, opening Monzo, letting the transaction list update, then checking the data usage stats again. It went up by about 1.5MB. I took a screen recording as proof!
So I am now fairly certain this is because I’ve been a heavy user for a long time and I have a lot of transactions. I expect this will affect more people as time goes on, so I’ll send a message to the support team about it tomorrow. Hopefully it can be changed so the app doesn’t download the entire transaction list every time. Thanks everyone who responded.