OK I have just found it in my XP Pro system
C:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\URL.db
Or C:\ProgramData\AVAST Software\Avast\URL.db in later OSes

But it is no where near the size yours is. Mine is a little over 38MB, even so that is relatively large, it was created on 1 December 2015 and last modified today. On my win7 netbook which isn’t so heavily used it is only 8.4MB and on my Win10 laptop it is only just over 6MB (but this is only 10 days old).

I’m not sure exactly what this is used for, at first I thought it could be related to checking DNS locations to confirm them. But why they could grow to that size is not good housekeeping.

So there certainly appears to be a problem.