The question is interesting and relevant. Athletic girl is fast, robust and strong in relation to its weight. Fat girl, however, is stronger in spite of the slow. At the beginning of the fight slim girl must quickly attack kicks and fist blows. This may weaken the fat girl. But fat has a strong punch. Athletic girl will have to be avoided punches. It threatens to knock-out. In the clinch and ground fighting chance to grow a thick girl, thanks to her strength and weight.
So fat girl is not physical fitness. It probably did not acquire combat skills. The fight will end when the fat girl will try to block before kicks and blows with athletic girl fist. Then the slim girl grabs her by the hair and rolling over to the ground. Then she will be beat with fists and kicks.
Athletic girl must have a good sense for the fight to win in the clinch and on the ground.
Conclusion: The chances are almost equal. The fight will end before the two women are on the ground and win a athletic girl. If both will fight to the ground then win fat girl, but this is less likely.