Lately, when I open the profile browser with Large previews selected, the preview images are often not large, but very small. If I switch to Grid or List view and back to Large, the preview images get the correct size and fill the width of the side panel.
This often happens the first time I use the Profile Browser after LR has started up, but not always, so I'm not sure what triggers this behavior.
Today this issue was back again. I guess it is caused by the same bug as in this thread, because the temporary solution is the same: It helps to go from one collection to another, or, in this case, to switch from one view size to another.