The only possible way I can think of is ask the creater of the model to remove details or something else that reduces file size. (I don't know what makes the file size of a MDX bigger or smaller). You can also ask if any other modeller can do it for you, but if you use a model made by someone else then Blizzard, you might need to ask permission of the creater to edit the model, and credits may still be given to him if he wants to.