You need to be a little more detailed as to which sub-folder it is, two of concern are:
1. the defs (definitions) sub-folder as VPS updates occur you effectively have a new sub-folder for each, avasts housekeeping should eventually clean old defs sub-folders. Each one is if the order of 131MB, so it doesn't take long for that to build up before it amounts to a large figure.
2. the setup sub-folder, this is a pretty big and essential folder, but I'm sure it could do with periodic housekeeping by avast.