Thank you for the reply. In the end I've managed it. I scaled the model in Blender, exported it and imported it into Maya. After that I imported also the skeleton and deleted the added animations. Then I changed the skeleton and made the weight paint. After that I imported the animations again.

Now it works in the game again...
But thanks again for the reply, it's kind of hard to find the problem if there are no teachers who can help me...