In order to get a fatter butt can you eat a ton of junk food and sit around and then when you gain weight do situps or will the situps decrease your butt fat too?

No, you cannot target a specific spot to lose, or not lose weight. When you lose weight, you lose it over the whole body.

But why don't you try some cardio exercises to lose your unwanted fat and you can have a healthy heart too!

Situps are used to build ab muscles rather than burning belly fat.