The Alone show is pretty interesting as an example of what could happen being in a box with no food. Men and women of all shapes and sizes go into that show and they all just slough off weight, like over a pound a day sometimes, and struggle bringing in enough food for their body to not starve. Many are medically discharged from the show if they lose too much weight too fast where it puts their internal organs at risk.

There's a psychological component to sticking to a diet for sure, but no, a calorie deficit of the same amount will result in the same weight loss rate.

People locked in a box will not be able to cheat and will demonstrate this.