Not sure why nobody has said it, but one of the main reasons to have larger port area is port noise. If you use too small of a port, the air will have to move faster through the port since the area is smaller, and it can cause a chuffing sound and even hinder performance of the sub. It would be similar to sucking through a coffee straw vs. a regular straw. If you have x amount of air to move through a port, it will have to move faster through a smaller port to get it all through in the same amount of time (one stroke of the subwoofer's cone). If the port is too small, all of the x amount of air may not make it through the port in time before the cone has changed direction, and now that amount of air that didn't make it through is pushing the opposite direction on the sub's cone, which is trying to pressurize the box in the opposite direction. Larger diameter subs and those with higher xmax need larger port area because they displace more air with each stroke So while your walmart sub may only have 8 mm xmax so it may be ok with a 3" diameter port, an SSD has three times that amount and would likely need at least one 4" port or larger.