Why is my disk space shrinking?
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.