its not really that the sub thinks the box is bigger....it changes the acoustical properties of the enclosure.
when you stuff any box full of fiberous material it slows the backwave. in doing so, you create better low-end frequency response. the amount the backwave is slowed is directly proportional to the amount of stuffing used. If your box sounds like crap below 40hz, pack that sucker full. if it gets a little response but not quite enough, just use the suggested 1lb/cuft.
Advantages of large sealed box over small sealed ...
- Less cone distortion
- Better transient response
- More accurate
- Louder low-frequencies
- Smoother roll-off