Shiny new tanks don't hold more gas. I was a little bummed when my new tank got swapped out, but whatever. They swap an aluminum tank for an aluminum tank. And no one really cares what your gas tank looks like.
On the flip side, I was happy when I swapped a steel 15 lb tank and all they had to give me was aluminum. I traded up on that one. But again, whatever. Next fill, I'll get whatever they have.