It seems that a good explanation of "and then some" is:
and then some
(Informal) With considerably more in addition: This project will take all our skill and then some.
Is this a quite accurate definition? (with the stress of "considerably more"). Why is it like that, I wonder -- is it a short form of something, such as "and then some more" or "and then some considerably more amount"? Actually, just from the word "some", it seems it is a little more or somewhat more, without the meaning of "considerably more".