It is a myth that the Yorkshire Terrier was once a larger dog. The first purebred Yorkies were bred to be small. The whole idea of the dog was to fit inside walls and other small areas to catch and kill the rodents. This also accounts for their very feisty nature. It's true that some of the dog's used in the beginning of the breed may have been a bit bigger when the breed type was set it was a small dog.
The famous little WWII war dog hero, Smoky, was a Yorkshire Terrier under 5lbs and fit in her owners helmet. It's a great story:
Smoky The Yorkshire Terrier Was Probably The Cutest Service Member In WWII
and proves that the Yorkshire Terrier was not only small but very sturdy even back during WWII.
Sadly, some bad breeding practices have changed both the frame and health characteristics of the Yorkie.
I hope the OP's new pup proves to be a very sturdy specimen.