That sounds very interesting, and would have been just down my alley... and not even for big boobs.
There was a series by Gordon R. Dickson, where a modern day guy is transported to a magical version of medieval England, in the form of a dragon. The first book in the series came out in 1976, and the second in 1990... but there are no big breasts featured in this series.
Piers Anthony's "Xanth" series was already going strong in the 80's, and these do feature well-endowed women... but none of that size or description, and the setting is completely fantastical.
Terry Brooks "Landover" series started in 1986, and also has the main protagonist move from the modern day to a pseudo-medieval kingdom... but no big boobies there either.
So, sorry, I can't help you here. But maybe someone else has any idea.
I seriously hate that problem. There's a number of books I read in my childhood (nothing boob-related) that I would love to revisit, just for the nostalgia, but with just a snippet of recollection about the story and no further informations, it's almost impossible to find the specific book.
Best result I had in that regard (again: not boob-related) was a series of English books about the adventures of a boy at a british boarding school. Took me ages to find, because for some reason they had changed the protagonist's name in the German versions, and it just didn't show up on any search back then.