They are both 13.The martial artist is a girl with 6 years experience and is over weight or a guy who has about 3 years experience with wrestling and is just a little overweight

How are they fighting?

It could go either way. First you would have to know what discipline of martial arts the girl is... that would help to know. If the fight remained as a standing fight, the wrestler would probably lose. If the wrestler got a hold of the martial artist and was able to get her down, chances are she would lose... that is unless she had training in a discipline that also deals with grappling and ground fighting.

A martial artist is not always a good fighter unless they spar, next a martial artist will do point sparring, so they do not pin you to the ground.

Whose rules are you using, if the wrestler has to use the martial arts rules he will most likely lose, since he does not do point sparring.

If this is a real fight to hurt each other, the martial artist if they are trained in fighting, since they will kick and never let the wrestler close enough.

The issue you are too vague, in martial arts you may never fight, I have students who have studied under me for 4 years and they do forms, they win national titles in forms and weapons but have never fought. Martial arts is not about fighting it is about style.

Next what type of Martial Arts, Judo ? Akido ? Karate, TKD, KungFu, Kenpo or Kempo, each style fights differently

Seems like your using street fighting rules. I studied mixed martial arts with professionals to help my boxing ,and mostly the people just taking classes were practicing stretching, some slow drills and how to keep a level head. I trained their for 6 months and they teach karate/Judo to kids only 3 times in the 6 months did they do light sparring. The only people doing full sparring regularly were the 2 pros that own the place and a group of 10 of us that were all already experienced fighters.

Now lets go over to wrestling. They do full drills full contact sparring constantly (even kids do full contact practice).

So at this age I would put my money on the boy because with martial arts even the 14 yr old black belts sparred and got hands on practice very rarely.