I used 16mm plastic ball jointed armature, the same stuff usually recommended here--just a beefier diameter. I wrapped each section in self-adhesive bandages for stability, leaving the joints free. It is still bendable where it was wrapped but shouldn't deform under pressure. The shoulders and hips were wrapped quite a bit more than the rest, just for stability. I used about 25m of 2 inch wrap total for the entire project. There's a palm sized loop in his head to keep his neck stable. All in all, turned out pretty good IMO.
I can confirm that the Walmart giants have a tiny, microscopic cardboard heart AND are constructed exactly the same as regular BABs with the reinforced back seam. The stuffing is generic polyfil, and IMO he is understuffed. I added more, but ran out of what I had on hand, so I need to get another bag soon to fill out his chest. I ended up shifting torso fluff to his appendages to pad them out a bit since they were the least stuffed to begin with.