They say, eating too much keeps us away from doing good deeds. Sometimes I need to eat much because I'm very weak. Is it a bad thing?
Eating too much, which is disparaged in some narrations is a relative concept. It changes from person to person. A big meal for you may be not enough for another. The disparaged thing is to continue eating after you become full. This is forbidden as it is a harmful and a wasteful attitude.
Allah the Almighty commands:
“Eat and drink, but be not excessive. Indeed, He does not like the lavish.” (Al-A’raf/The Elevated Place 7:31)
This is why the Messenger of God advised to reserve one-third of the stomach for meal, one-third of it for water, and leave the rest empty to breathe good. Related narration is as follows:
Narrated from Mikdâm b. Ma’dikerîb, I heard that the Messenger of Allah said:
“Human beings do not fill a plate worse than their stomach. It is enough for him to eat and drink only as much as that keeps him up. If he has to eat more, then let him reserve one-thirds of his stomach for meal, beverage, and breathing.” (Tirmidhi, Zuhd/Ascetism, 47; Ibn Majah, At’imah/Foods, 50)
This is also what is advised by experts of our day for a healthy and balanced nutrition.