There is a formulaic expression in English, going back quite a while, that goes "the xxxxxest xxxx you ever did see. From Google books:
1666: the finest pile I ever did see — from Samuel Pepys' diary
1792: strangest reason perhaps you ever did or will hear
1799: the ugliest old creature I ever did see
1832: the highest tree prehaps you ever did see
1832: the drollest varmint perhaps you ever did see
1835: the primest piece you ever did see
1836: the most knowing-looking little bit of a horse you ever did see
1836: most genial islands you ever did see
1851: the greatest muss you ever did see
1866: we have got one of the gratis olde raskells for a Captain you ever did see
1884: the sickest little shaver you ever did see
1920: the best bunch of man-hunters you ever did see
1968: the biggest lock you ever did see
1973: the cutest thing that I ever did see
2000: the happiest child you ever did see
A substantial fraction of the Google hits for "you ever did see" follow this formula. My answer would be: there's no significant difference in meaning here; it's just an instance of this formula.
From Google books search, it appears to have increased in usage fairly abruptly in the 1830s. Did it really, and if so, where did this come from? The only thing that I can think of is the children's song. But there's no evidence this dates back to the 1830s.