Neither class was well thought of for this. Being fat was taken as a sign not of status, but of idleness, sloth, greed, and gluttony.

So, nobles, merchants, monks, and millers were all more likely to be fat than farmers, and all were seen by farmers as idle, greedy, gluttonous sloths. It's not clear to me why you think that means farmers saw them as having low social status, rather than resenting their high social status.