There are so many factors and variables there really isn't anywhere to start explaining because, frankly, it would be an answer that is pages and pages long. But I'll try and cover some basics. SQ and low xmax go well together for a couple of reasons. 1. Because SQ drivers aren't made to get very loud, they are designed to be accurate, so high xmax isn't an extremely necessary thing to have (keep in mind high xmax does NOT always equate to loud). 2. The higher the xmax of a driver, the harder it is to make sure the movement of everything remains linear once the driver starts to reach its limits. Here's a bad analogy of what I mean: is it harder to ride a bike in first gear and have to peddle very fast and uncontrollably, or is it harder to have it in 4th or 5th gear and have a much more smooth motion.
High motor force drivers by nature are great at SPL because of their ability to force the cone back and forth dare I say, violently. High xmax drivers on the other hand don't usually incorporate strong motors, because instead of relying on a strong magnetic force to generate SPL, they rely on moving a lot of air. Because of this, high xmax subs are great for setups that are geared towards moving a lot of air and, consequently, doing great on low end authority (also keep in mind that high xmax does NOT automatically mean the sub will do great on low notes. This is a combination of many other factors as well as the enclosure it is in).