Reasons why you might faint during sex: (keep in mind I’m a vet not a human doc)
1) Hyperventilating with not enough oxygen getting to your brain
2) Being super anxious and having something called vasovagal syncope, which causes a sudden drop in blood pressure.
3) pretty much any heart condition which affects oxygen during increased heart rates.
These reasons don’t have much to do with the size of doggie knot, and my guess is this person probably had some other issue.
The key is training yourself, and your ass, before the actual event so that you can relax and enjoy it. Zooville has great resources for how to do this, look in the “How to’s and educational” section.