It's kinda weird that it's so much bigger than the std cab window (same width and shape just about 3 inches taller) and I like having the bigger window. I found out the same way after trying a std cab window in my Xtra-cab. 

The only reason I can see, as to why the std cab window is shorter, is because of where the head rests on the seats would hit if they're reclined. If the std cab had the larger window the seats would hit the window. Not good if the seat back latching mechanism should break if you ever got rear ended. 

On the other hand, you kinda wonder why they didn't just use the smaller window on the Xtra-cab. Maybe it just didn't look good.  kinda  if you ask me.