Jim,
And a Happy New Year to you.
Welcome, Sheena.

Funnily enough, I don't think the people who coined the term 'big ballads' were thinking of the comic ones. Personally I think Child only included some of these as samples of what was around, as they are no older and no more ballad-like than lots of others he could have included, Marrow Bones for instance. They are just as likely to have originated on broadsides as anywhere else and most of them have their earliest versions on broadsides. (Sorry, Jim. I know you probably will balk at this.)