Just to clarify the behavior here, the filter Leo suggests will filter out individual records (images, scripts, etc) where the size is less than 512 KB. So this filter may exclude more sites than you expect as many resources on a website page may be less than 512 KB.
For example, if a webpage is made up of 10 x 500 KB images, then this will give the site an aggregated size of 5 MB. But as this filter will exclude all those files from the report, the site will not appear at all in the report.
Our *Activity Reports* naturally filter out sites with a small amount of traffic logged, which you can toggle on/off using the 'Show more activity with very little size logged' toggle at the bottom of each hour group.
We don't have a way to remove sites with a total aggregated size less than 512 KB in our Overview Reports at the moment.