There's is a point where you can't easily tell what a fat fuck weights with only his gunt as a reference,he can have big legs and weight 300 or chicken legs and weight 260,but the thing is,you want the big legs.
The legs may be the most important muscle to burn calories when you are obese,that's why every ex-fatty in the gym has two tree trunks instead of calves.Ralph has two little atrophied legs because he never played a sport when he was young and he is always on the computer or seated.
Tl dr: Ralph might weight around 270 because he has the legs of a little fat kid on a obese man upper body.