Who would have won? Kovu or Simba?

[The Spirit of Life]

Simba is a lot bigger than Kovu. I'd imagine it would play out a lot like a human teen trying to fight a fully mature, physically developed adult man. Simba would win rather easily. I think that the only lion in the TLK franchise who could beat Simba in a fair 1-on-1 fight is Mufasa (edit: and Nala).

[Aelwen]

But Zira beat him up. Of course Simbsa had already been weakened before that fighting with lionesses but still...

[SimbasGuard]

Zira did not beat Simba up. She knocked him over, by catching him by surprise when he was off balance. Simba got back to his feet and was ready to fight in no time at all.