I found out in my VS 17.12 has the same behavior and it turned out that auto update cause these files (pre downloading all necessary packages and msi). I once got 7GB of files in temp folder and 5GB created by VS (mainly packages in Microsoft.VisualStudio / ASP / SQL namespaces, so it's all pretty sure about VS itself), and after I start updating the VS, the download size is exactly 5GB and the update is quick, and after it finishes, large files in temp folder are gone.