The GSD standard is there for a reason....oversize GSDs are fine and all but simply do not conform to the standard set forth. Perhaps an oversize GSD is not as likely to perform the physical tasks as the breed was originally designed for...I have to laugh at myself saying this...as I personally believe the GSD has been incredibly bastardized over the decades since 1899. I am a good example....I have owned coats...my preference...but until recently a coat was not in accordance with the standard ( German )....and now it is accepted.
Nonstandard size, coat, color etc is nothing new within the breed and most likely will continue to dilute the breed ...as long as there is a market for the$e dog$.
Perhaps, the breeder who objected to the larger version is more of a purist and strives to breed to the standard rather than a permutation of the standard.
As far as health and well-being of an XXL GSD, I am unsure...hopefully there are others who will be able to give you an honest answer to that question....and it's a great question....because who wants a jumbo GSD if the odds are higher that you have say goodbye all too early....simply because of it's size. You should get the dog you desire and from your question..you put health and wellness as a priority....good for you!