Why is my disk space shrinking? I've just got a second hand MacBook from my dad which is a 2017 version with 250GB storage. I've factory reset it so theoretically it should be a completely blank slate, but for some reason I'm losing disk space already. Every couple of minutes the available space shrinks; it seems I lose 100MG every ten minutes or so. I'm currently on 231.52GB free, and it was at least 235 a few hours ago.

I know this began as soon as the computer switched on because the first thing I did was to check the storage space and I immediately noticed that it was going down. I'm not downloading anything at all from the internet, my downloads folder is completely empty. 

This is really frustrating to me, not to mention making me a bit paranoid about when it's going to stop and how much space I'll have lost when it does. The whole reason I got this Mac was for the better storage as my previous one was less than half the size (120GB) and it was causing big problems. And granted it's only small chunk of the overall storage gone so far, but I'm gonna be using a lot of big files on this machine and I need all the space I can get. Does anyone know why this is happening and how to stop it? I'm wary of even downloading anything else until I get to the root of this problem.
I think having 235GB free space on a 256GB SSD Macbook Pro is not very likely on an active system. 256GB SSD Mac normally has about 250 GB of available space. The system and pre-installed apps would easily take much more space than 15 GB. 

On the other hand, mac storage reporting can dynamically change (up or down) as calculations take place. You might have been observing that going up and down in calculations. 

On a fresh installed OS, much things can be going. For example, spotlight might be building an index. As you browse web-sites, Safari or Google chrome will also be building up its cache. Likewise, if you use Spotify, even it will cache the songs you are listening. Therefore, I suggest that you use the system for a while and then see if the storage calculations settle down. 

When the things settle down, you can make an analysis of used space and try to see if everything is normal or not. Hope that helps.