Personally I would order something different as I wouldn't want to give someone something I don't like.
But as you said this nappy might be fine for your secret santa. Up to you really.
Sorry not much help
As you said all babies are different shapes. Do you know if Lockie's shape is the same as your recipients? Do they both have fat or skinny legs? Even if something was a brilliant fit on Lockie doesn't mean it would be perfect for another baby. I'd say not to bother getting something different unless you thought that your recipient has a similiar body shape to Lockie.
mmmm very tough. If you think its just generally a bad fitting nappy (ie very loose or badly designed) then I'd return it and get something else. If you think it might be ok on the other body shape then keep it!