What's the difference in meaning between these sentences?
You should eat less.
You have to eat less.
You must eat less.
In the context of eating less, you must eat less is a very strong statement. I'd assume it means: unless you starts eating less, you are putting your health, your well-being, and maybe even your life in jeopardy.
In contrast, you should eat less is not as emphatic. It means the person would gain some benefit by cutting back on their food intake.
As a footnote, in general, I think must reads a little better than have to in such constructs, even though they mean the same thing.