I (21f) am 5'5" and skinny. I recently attended a self-defense class, and there I was tought how to use a pocket knife. And as I have some male friends 6'5"+ tall, I thought that that height actually makes their belly particularly vulnerable in case of a fight (compared to a short guy/girl):
- a short person's shoulder height is around the same level as the the tall person's belly making it easier to stab/hit;
- a tall guy's belly is proportionally a larger target.
And in general, if a 6'5"+ guy stands in front of me with his stomach released (even if he has just a bit of a belly) it feels like having an exposed, large and squishy punching/stabbing bag right in front of me, into which, if needed, I can easily harshly and deeply plunge anything straight away.
I also practice historical fencing (rapier) and most of the times I stab tall opponents into the belly - my favorite thrusts are ducking counterattacks when I duck while they attack, go low and impale a tall guy's unprotected belly right onto my rapier.
As someone else who is short and skinny, this is a very dangerous assumption. I've trained in judo, karate, and wrestling, and gotten in a couple actual fights, so have some experience here. In a real fight for your life being shorter is a disadvantage, even more so if you have average length arms for your height. It's possible to overcome that disadvantage if you are more skilled than the opponent, but assuming equal skill you are likely to lose.
In an actual fight, going for a gut stab with a knife is one of the worst options you have. It means you are absolutely within grapple range, have done almost nothing to hinder their mobility, and even if it is eventually fatal, it often takes a very long time to die from a gut wound. You would be much better either going for a hit that would instantly kill them, or aiming for somewhere that would hinder their mobility enough for you to escape.
Also, in the real world it doesn't work like fencing, no one is going to break the fight up as soon as you score a hit. If you want to actually prepare for a fight you should try out a sport that doesn't break on point scoring. In general boxing, MMA, or wrestling would be a better option to prepare for a real fight that doesn't get broken up.