Then open a ticket. I found similar one, but nobody checked there why folder is so big: https://github.com/rullzer/previewgenerator/issues/129
P.S. It turns that changing of config.php will not automatically recreate smaller/bigger previews. This is how to do it https://github.com/rullzer/previewgenerator/issues/94#issuecomment-355745556 run pregenerate command afterwards.