It's the size and growth rate.. the French Whites were bred to grow faster and get larger than normal muscovy.. so where a regular muscovy will get to around 14 to 16 pounds for the drakes.. the French Whites can get to 18- 20 for the drakes ..
There is a "saying" about the French Whites.. 12 pounds by 12 weeks of age (again.. that's for the drakes with hens being smaller)..
*edited to trim down the post a bit by removing the extra quotes
Ah - got it. It's all about the size.