D..
I think that something is wrong with your Keurig. It doesn't do that. You shouldn't have to use 2 smaller cups to get one.
When you put the store bought cups in, are you making sure that the black cup thing is in there to put the coffee cups in? You have to take that out to put the little basket with your own coffee in. If you forget to put the black holder back in, the coffee will be very weak. (Yeah, I've done that...)
Anyway, I think you should take the Keurig back and get another one.
Dawn