Any suggestions why one of my musk turtles is a lot larger than the other?

It could be based on the turtles' appetites or it could be a difference (females are larger). You might try to vary the food a bit to find something the smaller one likes better. Earthworms, bait fishes, and liver dusted in bone meal are all good. To avoid fighting, get a tank with the largest bottom area you can. Stiff-sided wading pools are great. Put in either one large basking area or two smaller ones. A slab of wood held at a slight angle is possibly the best. Musk turtles climb quite well to find basking areas.