I'm 5'3 in my Sketchers (which add height). Lane Bryant and I only get along from the waist up. I still mourn the passing of Fashion Bug who had jeans that would fit me. I bought all I could when they folded and actively look for my size on Ebay to get spare gently used or new pairs in the style I liked.When I hear the term, I immediately picture clothing items at Lane Bryant, where my 5'4" wife has often expressed frustration with designers of plus sized clothes because "not all fat chicks are 5'10" and taller!"
The term to me conveys images of tall women with larger features. I dated one briefly years ago. She was over 6' tall and had big everything. Big think torso, arms, hips, butt, boobs bigger than I knew what to do with! She also wore heels when she and I were out and about so the size disparity was greatly pronounced. Shorter girls I've dated have said plus size applies to taller women.
I'm not sure if there is a rule written somewhere formalizing this claim, but the designers at Lane Bryant seem to agree.