very fat goldfinch — could it be moulting or disease?

Hi. I’ve seen a very fat goldfinch on my feeder today. ive never seen one like this before and i thought at first it was maybe just a young one all fluffed up and moulting, but when i got my scope on it it looked like his body was actually fat (abnormally fat). from what i could see his head looked fine, and he was eating okay, and he is the only bird that looks like that.

as a precaution i have removed all the food and the feeders and scrubbed the whole lot, and will leave it all completely empty for a week

im just wondering if it might be something else other than disease, and i’m worrying for nothing