What are the files in C:\Program Files\AVAST Software\Avast\setup for?
They take up roughly 50% of Avast’s install folder and are mostly .VPX, a few .EXE and couple .DLL files.
Note: I have no issues with low disk space. 
The .VPX files if my memory serves are used for integrity checking and repair function.
A search on that file type brings many hits relating to avast usage. https://uk.search.yahoo.com/search?p=file+type+.vpx