Tina
Matty Groves is probably 'little' because the sympathies of the ballad makers quite often lay with the 'little guy'. This doesn't always apply, the little guy can be the villain, as in some versions of this ballad where the one who grasses him up is 'a little foot-page'. But you will notice that here Matty is also the victim, having been induced into the bed of Lady Barnard by authority of her social status.
There is an interesting note to the ballad in Coffin and Renwick's 'The British Traditional Ballad in North America' where it is stated that some traditional singers considered it 'dirty'. If you can't get hold of a copy I'll be glad to post the note.
The main characteristic that distinguishes most ballads from the songs is the economy of language i.e. not much description, mostly straight-to-the-point action.