Watch the fight between Wolverine and Mystique carefully, and you can see her switch back and forth from the willowy Romijn-Stamos to a larger, more athletic stunt double. The woman who holds the section of fence while Wolverine slices through it, and then kicks him away, is very clearly the double.
Duh. How many stunt doubles do you think they can find that are above six foot and under 110 lbs? It's not a slip-up, it's a fact of life. If a woman looks like RR-S, she's a model or a trophy wife, not working for scale as a stunt-double.
Plus since Logan is bigger and stronger than Mystique then using her ability to change her body to become a bigger and better fighter might just infact be a good idea .(i know the size change is due to the fact its a stunt double,, but this is a perfectly good reason)