I use a Dell Laptop Vostro and I'm a paid-user of the McAfee Security Center.
A folder in C: ...\ProgramData\McAfee\VirusScan\Quarantine\... is about 5 Gigabyte big.
Why is it so big?
Therein are about 400 files with a BUP-extension. Whatfor is a bup-file?
May I delete them to save place in the harddisk?
BUP, per a Google Search to verify, signifies BackUP files, usually things like movies or music etc. It's OK to delete from the quarantine folder.
If you feel that perhaps it is a wrong detection read here: https://community.mcafee.com/thread/2016