If you really want to work with ML and make models for mobile. Then you should consider this:
1) Do not use such systems like Watson Studio, and use instead Tensorflow framework and train models manually with understanding what you are doing and why. Here is lib for converting TF model to CoreML models https://github.com/tf-coreml/tf-coreml
2) Use weights optimization technic, to minimize your model size and increase performance. Cause just loading 500MB models on phone is not a good thing, and people who will burn their hands with phone, will not thank you.