straight from www.diysubwoofers.org
1.04 - What happens if the box is too small?
If the box is too small will typically result in a boomy system that appears to have strong midbass and less low bass. Typically power-handling is improved though, unless the box is REALLY too small! You can compensate somewhat for a small box volume by adding stuffing to the box. The stuffing can make the box appear up to 40% larger to the driver. Note that sealed systems are generally a lot more tolerant of variation in box volume than are the other tuned-port systems such as the ported and bandpass systems.
and linked for the lazy