Depicted here is what happens when Mettaton hears the term "fat and fabulous" once getting to the surface. The harsh reality that he cannot physically appeal to literally everyone is just something the vain automaton cannot accept and it's not long before he's regularly forcing poor Alphys into constructing new bodies for him with silicone padding and other such added "equipment" to feed into any possible body preference that could exist which all quickly spirals into an crazed addiction filled with scandals and interventions, you know, just like any true celebrity!

Okay so, I may have put WAY more thought into this then I should but honestly I could see this happening... okay maybe that's just me wanting to see Mettaton put on weight because I think it's hot but I figure if Undertale can have a canonically overweight skeleton, then the robot can be fat too!