this happened before already, when there is a bfga2 update, like this current update we have now which has 204.4 MB and i have more than enough GBs in my disk for it and when i try to download the update it keeps saying NOT ENOUGH FREE DISK SPACE. there is also the issue when sometimes this update tries to take up much more space than it needed.
The game does some MASSIVE file shuffling when doing updates. You are going to need a few GBs on your disk for steam to be able to update it. Most of the time an update that takes up 50MB can take up to an hour to reshuffle all the files. It does depend on your drive though, SSD should do this fairly fast.