Mac Mini System Data at 276GB?

Hi! My Mac Mini has a 512GB SSD. When I look at the space available it says i have 494.38GB of space available, but 276GB has been taken up by "System Data"? That's over half the space!! How do I reduce this?

I am running Dropbox and have selective sync turned on, but when I look at the Dropbox stats it says it's only using 60GB of Dropbox space, and then 388GB is "Other hard drive space used"