While it's not compulsory to finish your plate in Italy, leaving a significant amount of food uneaten may raise eyebrows and be perceived as a lack of appreciation for the effort and care that went into preparing the meal.
However, it is important to listen to your body and only eat as much as you comfortably can. If you find yourself unable to finish your meal, it is polite to apologize and explain that you are full. Most Italian hosts will understand and appreciate your honesty.