When clicking on the parent of a group of feeds, the combined list of feeds is 500. Is there a setting somewhere to control this? Individually, the feeds can have thousands of posts displayed.
In windows, find RSSOwl.ini [for me, it's in C:\Program Files (x86)\RSSOwl] and add the line -DnoFolderLimit=1 at the end. The same setting probably exists on Mac and Linux but I don't know where to set it.