I've googled the phrase "and then some" and am told that it means "considerably more".
But just how to comprehend this? The phrase literally means "some more" -- how does it come to mean "much more"?
I think it's a case of using understatement.
Taken literally, "and then some" could be taken to mean "plus a little more".
But the way it's used, it generally means "plus much much more."
And then some is used only when X exceeds Y by a significant amount, not when the margin is small:
So whenever and then some is used, the difference is already ‘considerable’. And then some adds even more (which is what the phrase means), so the sum of the two margins is, as you say, “much more”.