I'm so glad the Vet was so confident, breeding based on an old wives tale instead of firm, experience is always
such a good idea
If a mare will only carry a foal that she can foal out, how come we get so many dystocias with big foals??
Larger mares are a different matter, I bred smaller Welsh mares to our Arab PB Palomino, no problem, and he was 15.2hh.
The mares never had problems.
Would I have bred these mares to a Draught horse?
No, no way whatsoever. Ever!
I suppose it comes down to whether or not you care if the mare dies giving birth, If you do not, as that Vet obviously did not, then you can afford to let luck take it's course.
With Minis you have much less room for error, and the size of the mare does not govern the size of the foal, so I would always breed a small stallion to a larger mare.
And, of course, I would never breed and unregistered foal, anyway.
What would be the point?
But to answer the original question, there is nothing you can do, it is neither your problem nor your business, so I would stand back and be glad it is not your problem