I don’t suggest buying clothes for the gf. One, I agree that buying her plus size clothing might rub her the wrong way if she views it in the way you speculated. Two, if they break up, it’s a reminder to her of the mother in law that didn’t happen, making her feel either sad, apathetic, or relieved. No option is positive.
I have never purchased clothing as a gift for my son’s gf of seven years for that reason.
I mention this because my D’s ex-boyfriend’s mom bought her clothes and she will never wear them again, for one of the reasons mentioned above.