What materials are best for minimizing the internal volume of an sub enclosure. I have heard dense expanding foam like Great Stuff works best and that some other materials like pillow stuffing can have an opposite effect. Is this because the non-dense material traps sound waves instead of reflecting?

Also, why is there no concern regarding the internal dimensions for enclosures on smaller speakers, like the 6.5 in the door and rear? These speakers when upgraded can produce pretty low frequencies and I know mine definitely put out some bass. Is it also important to minimize the internal space in the door and rear speaker spots?