Summary:
There is more reason (in the הו"א) to assume that רקיק is not ממעט, since it can fall easily and he is not מבטל, as opposed to עבה where only the second reason applies.
We see; whenever if they would be removed - רואין כל שאילו ינטלו
Summary:
A דבר החוצץ has to be able to stand on its own either because otherwise it is a סתימת ארעי, or because (if its support is a דבר המקבל טומאה) it cannot be חוצץ בפני הטומאה. Others maintain that even if the support is not מקבל טומאה but it is a כלי, anything that it supports cannot be חוצץ בפני הטומאה.