I think the weight difference is probably a combination of thinner metal, and LESS metal (as it's a few inches shorter).
Thanks for these pictures; especially the comparison with the old JJ/SJP, which I also currently have. I added a three hole damper in the bottom of mine for better airflow and it works terrific-ly for me. It'll get hot enough to sear burgers and steaks, but won't stay ultra-hot (>500 degrees) for hours on end. I have two kamados to make up that slack, so I'm OK there.
The new Jumbo Joe reminds me of a 3 1/2 inch wider version of my SJ silver, which works GREAT for high temp cooking. I'll pass on spending 70 bucks on the new one, and will wait for them to show up on CL.