Bloated Windows 10 Installation

I struggled mightily to transfer my Windows 10 Boot Camp installation to a new Mac Book Pro using Winclone, having to inject drivers and I don’t know what else. It has been running fine for months meanwhile. The problem is that I have almost no data of my own on that machine and Windows reports that “system files” are taking up almost 2 TB of space. Why is this and what can I do about it? Thanks.