Conversed: "Yes, I'm skinny. No, that doesn't make me a bad person- what is this, a movie?"
Deconstructed: Overlord Smith spends so long obsessing over his evil scheme that he never eats; his health is affected dramatically and he begins to suffer from Sanity Slippage.
Overlord Smith realizes that failing health will not help him accomplish World Domination, so he adopts healthy eating habits.
Being a beanpole is just Overlord Smith's natural build. He eats well, but his metabolism burns everything it gets.
Played For Laughs: Overlord Smith's Jewish Motherlives with him, and interrupts a dramatic confrontation with the protagonists to nudzh him to eat some noodle kugel, claiming she can see daylight through him. "So you weren't going to let me meet your new friends? They can have some too."