Miniskirts have nothin' to do with it--they weren't invented when the phrase was well-established.

If they had meant long-legged, they would have said so--it scans the same. A BIG legged woman is a lass with legs that look big. The appearance of this phenomenon in the field of view of the average male often causes him to ponder on other architectural mysteries and God's impeccable mastery thereof. Especially proximate ones.

A