On my plate style it was the HAZ (heat affected zone of the weld) on the plate that cracked and broke, basically the weld made the metal plate brittle and it cracked. The 7/8" tube probably wouldn't tear so easy. May have just come up with an analogy:
flat peice of paper, lift up on the edges as it sits on a table, bends easy, right? Now roll that piece of paper into a tube, lift on its edge, and it doesn't bend up so easy.
As for the chromo post, chromoly is pound for pound significantly stronger than mild steel. So if the chromo post is the same thickness as a stocker, its wwaayy stronger, or it could be same strength but quite a bit lighter. I would imagine the chromo post strikes a balance to provide alot more strenght than stock at a reduced weight.