Penelope Ody ( English herbalist) lists the use of cabbage leaf poultices (
beaten slightly to soften and rib removed ) for arthritic or sprained joints,
varicose veins, wounds, and for mastitis and engorged breasts.

What does a cabbage leaf suggest to you - a receptacle to be filled with ?
Yes, indeed, an engorged breast would just about fill the ticket.  The
doctrine of signatures  may have been amongst the earliest ideas that humans
had about plants - it looks like [body part] - gee, maybe it would help with
this problem that I have with [body part].  Would have made quite an
impression if the plant then did indeed have a healing effects.

Of course, this basic idea did reach the most elaborate proportions in some
cultures ( e.g. historic European herbalism ) and has left us with many
fanciful and sometimes curiously useful ideas about plants, based originally
on their anatomy.

Have no idea of the agricultural ancestor of cabbage but humans would have had
to develop cabbage for some time to have produced those breast-sized/shaped
leaves.  But perhaps cabbage was first applied to breasts and later found to
be more generally useful for other swellings and discomforts.

Sheila Humphrey