Some of them are just models and porn stars getting a bit more cash and maybe a kick out of seeing themselves on cinema, but I have a feeling that loads of British girls in the seventies were typecast and their careers never taken off because of it. But even though those films and shows may have destroyed ambitions of any big busted girl who was sick of being slapped in as a joke, I cant help but love those scenes, I know that loads of boys coming into puberty were profoundly grateful for them.
My brother recently bought that complete Monty Python set(all tv and film material, but none of the radio/record stuff) and was surprised to see some of these girls used in breast jokes. Not Carol Cleveland, who everybody will have seen, but other girls in what are actually pretty poor jokes by the Python standards.
Up Pompei was full of them(someone recently posted an amazing youtube clip), Carry On, Hammer Horror films(Hammer is actually getting revived, but so far things do not look promising, as a horror fan, we'll see if they can offer some compensation with buxom vampires). Even as recently as the 90s with the Fast Show. Linsey Dawn Mckenzie got used a few times too.
One of my faves is Luan Peters, who I first saw in Twins of Evil, which is full of gorgeous girls, you see her topless in it and her breast gets bitten into by a female vampire.
http://en.wikipedia.org/wiki/Luan_Peters Lots of the pcitures on wikipedia dont make her seem terribly big but she really stood out in her film and tv appearances I have saw.
Other things that spring to mind is Alexis May(was that ever cleared up if it was her?) jogging in Adam Sandler film Click. The mother in Me, Myself and Irene.
This of course does not include adult films or Russ Meyer flicks where the breasts are the whole point. I'm mostly referring to horribly dated jokes in cheeky comedy.