Yeah that's horse ass. It assumes that the bigger guy also didn't train to fight and that you can overpower him anyway. I took 5 years of Jiu Jitsu before my first fight and the guy was 6 inches taller than me and almost twice my weight. I couldn't get him to the ground for shit and he whipped me like a ragdoll.

No, it assumes you've been putting in work at the gym which it doesn't seem like you did. You should be able to take someone down at will after 5 months, let alone 5 years. The fact that he was so big should have worked in your favor. A sweep single would do it. The bigger they are the harder they fall.

Second fight I stabbed him with a swiss army knife and everything worked out no problem.

I hope that was self defense because I don't think stabbing people in street fights is taken lightly by cops.