Here's a few tips that may save a bit of space.

1.No one says the entire bike has to go into the same case,many parts can be removed and put into another piece of luggage such as carry on,etc.
2.Remove fenders Leave them home get a couple of plastic ones that roll up, the kind that attach to seat tube on back wheel and I think they make one for front wheel too.

3.Remove air from tires saves almost an inch or more depending on suitcase size and shape.