Good metaphor. One does not get suddenly obese, it happens gradually and steadily and is well correlated with overeating. Your metaphor illustrates my point, not yours.
If you've been eating the same for the last 20 years and you've been gradually gaining weight over the last 20 years, then it's your diet.
If you've been eating the same for the last 20 years and you suddenly gain weight, it's likely not your diet, it's something else -- exercise, illness, et cetera.