“I was training” is grammatically correct, but “I trained” is more natural. Also, you could say: “I want to gain more size.” I hope this is helpful.

I want to have/I want to develop big musclesも大丈夫 👍 If you want to practice English with a professional teacher, feel free to book a lesson with me. 日本語もちょっとできるからコミュニケーションは難しくないですね 😊