it was a fight between them and she won

Nala would pin Simba any day! :D

Nala: Ha. Pinned ya.
Simba: Hey, lemme up.
Nala: Pinned ya again.

We see it in the movie, that's why :P

If she did it before she can do it again!

Nala would pin him and win'em!

Back then in the jungle Simba was a lion who hadn't ever fought with anyone, was eating bugs to survive and used to live with a warthog and a meerkat. Nala had to fight a lot in her life imo with hyenas etc and hunt. This is why i believe she won simba in that fight. However Simba now is aware of Nala's pinning techniques and wouldn't tackle and pin her in the first place so that she wouldn't backpin him. He is a lot larger and stronger and if you remember how he could fight 5+ lionesses during the battle I think he can easily beat Nala now.