One reason for increased strength is load distribution. The 44 ring gear and pinion teeth are wider, meaning there is more surface area in contact. Specifically, the Dana 44 tooth measures approximately 1.401" at the base. The Dana 30 tooth comes in at 1.24". There may also be a slight depth difference, but that one is harder for me to measure.
A good example in how this matters wold be ice. If you walk on thin ice with just your shoes, you're concentrating all the force in a condensed area and raising the probability of the ice breaking. If however you put down a board and walked on that, you'd be spreading out the force and reducing the risk of the ice breaking.
To be clear, I'm not saying the Dana 30 R&P is weak. If it was, we'd see a lot of breakage, which we do not. I know several people that beat the crap out of their XJs and they don't break the R&P. Back to my original post on this, it's just that if you look at them side by side, the difference between the 30 and 44 is substantial. And, the picture doesn't really show just how dramatic that difference is.