You can restrict the size of report files, avastUI, Settings, Maintenance, Auto cleanup and the Maximum size of each log file, I have mine set to 64KB so that will drop off old entries. You could set it lower as 4KB for a txt file is still large if you are browsing it.
There are some files that don't abide by this limit, generally the setup.log and ones that are more benefit for debug/developer use.