"the" in the bigger, the better

Here was one thrown to me:

Why do we use "the"  in phrases like the more the merrier, the bigger the better, etc?

Let me try to throw back a quick answer. I see the phrase as a short form of  the very awkward 'The bigger thing it is, the better thing it is'. I don't think that here you would question the use of  'the', would you?

Consider also 'he's the bigger of the two boys', where the use of 'the' seems reasonable. ie the bigger boy of the two boys.

best wishes, Clive