VPS update on avast7 would be at least the size of the current defs sub-folder plus the increments.
The current size of the defs sub-folder is around 80mb, so I don't know what version of avast they have, if avast 7 it can't possibly be only 10-20mb. See image attachments for two of these defs sub-folders. As I have two defs sub-folders my over all avast size would increase by almost 80mb (image3), but after housekeeping that would drop back down.
So I really have no idea what is going on with your systems.