Rabbits are well made to eat plenty of grass. They have got the teeth and the guts to do a brilliant job. Their teeth are made for small blades of grass, lots of fibres wearing the teeth constantly down. They grow back and can grind those grass blades down again and again. The intestine works well if they get a constant supply of fibre. In their caecum, they can digest this high fibre low calorie food.
So the best food for rabbits is grass and dried grass, hay.
In the wild, for a few months in the year the rabbit will get corn and other seeds. Corn is high in carbohydrates and fat. Most rabbits have a sweet tooth and they are only able to eat this type of food for a few weeks because then the long hard winter comes. As a rabbit owner, we tend to give our pet rabbits a cereal mix of grains and sweet peas, and to a rabbit it is like a child in the chocolate factory.
Yes, they do also eat carrots and other vegetables, but remember in nature we have little carrots, not those greenhouse bred carrots.
So ideally feed your rabbit grass! Generally we recommend to feed 70% of grass or hay 28% vegetables (more the tops of the carrots etc) and only 2% of rabbit food and then ideally a pellet food so they cannot select all the sweet and fatty food.