To explain why 2" webbing is more effective than 3" webbing, one needs to look at the shape of the human body. The Iliac Crest is the part of the pelvic bone that sticks out above the hips. 2" webbing fits entirely within that crest, where 3" webbing rides over the tips of the crest. The 3" webbing has less surface area by percentage than the 2" webbing which will cause more bruising on impact as "hot-spots" of force develop where the belt is making contact with the pelvic bone. Since the 2" webbing rides inside the crest it can be worn as much as 1" to 2" tighter and at the same time is more comfortable.
Since the 2" webbing fits well within the Iliac Crest of the pelvis, it is less likely to slide up above the crest and cause submarining - a condition where the body slides down below the lap belt possibly causing internal organ damage.
Research shows that the faster the pelvis is captured, the lower the resultant loads on the chest, head, and neck. There is no difference in the strength of the 2" webbing. All webbing (2" or 3") must meet the same load tests.