The ones I've seen, and the one that came with my PDP kit are pretty huge and pretty much stuff the drum.
Even if it doesn't touch the heads, having something in the drum helps reduce the boingy pipe resonance. That sewer pipe fluttery echo thing. Currently I use an Evans pillow in the bottom of my Gretsch Renown, not touching either head. It takes up much less space than the DW pillow but is enough to tame the pipe resonance. If I want, I can put it up against either head but that usually isn't necessary. Maybe for recording when I get around to swapping out the SuperKick batter for an EQ3. With the Aquarian, I just tune it JAW for recording which comes out fairly dry, and tune it up to a pitch for unmic'd live gigs.