Gracie Bon's leading the charge for big booty travelers everywhere ... asking for more accessible seating while 30,000 feet in the air -- doubling down on her plea from earlier. We talked to the Instagram model on "TMZ Live" Friday ... and she absolutely torched the seat sizes on airlines as they currently exist, explaining she's comfy on trains and automobiles -- but there's barely any room for her on planes. Yes, it's a real problem ... so says GB!