Oh dear, I feel a spell of 'that guy' coming over me...
***Historically*** The distinction between weights of armor wasn't a major concern for fighters. The deciding factors were available technology and what you could afford. Plate armor can afford more mobility as well as protection than chain because it can support itself rather than going limp. Also if you wanted to be 'lightly' armored, you could wear pieces of 'heavy' armor on parts of your body you expected to get hit and not on places you wanted free. Hoplites did this and it seemed to work out well.
Then again, this isn't history, it's your-story. If you want to have armor be ;medium because it is fun, that's perfectly cromulent.
Last, if Paige wants to pretend to be a boy she might do better if she hid her 'jumbo potatoes'. They're not very 'plain'.