Physical Health in the Modern Sangha

[Metaphor]

Some of this has been discussed here recently:

Why are monks that eat only one meal a day overweight?

This question is not intend to turn this discussion ugly or throw out accusations but comes from a genuine curiosity as a layperson. Do monks become overweight due to a general lack of exercise or do they over-eat on that single meal time to carry them through the day? And how do monks avoid gastritis when they only eat once a day? And in general when monks eat once a day do they set out keeping in mind a certain amount of food they must eat to get through the day?