Why in case of a stab (especially if the person stabbed is significantly tall) - its the belly which usually gets wounded?

I (21f) am a pretty short (5’2") fencer (HEMA, rapier), and I often get advice to target a tall opponent’s belly with my rapier. Also I’ve attended a self-defense class, and the instructor gave me advice to go for the abdomen of a tall attacker, if I have a pocket knife in my hand.