Big breasts get in the way of the rest of the woman.
with 4+ if you even get hugged, you can only really be in contact with the top half, in essence she hugs you with tits and arm.
there will be a space lower down.
with DFC she will try to make up for the lack of tits, and you will often find her vag up against your legs, also your cock would be rubbed.
a big tits cant do that because it looks to slutty, wheras a DFC user can do that and it looks innocent.