Once you are used to eating a big meal it’s not that hard to put away 1600-1800 calories in one sitting. Then add onto that a juice drink, milky tea/coffee or some other ‘evening allowables’ and you are close to 2000kCal and that’s without taking any desert!
From time to time I will track my calories, by eye-balling (I am no longer the weirdo who pulls out their tri scales) and enter it into myNetDiary. On days when we have a big dāna offering it’s certainly possible to consume 2000kCal.
As we get older we no longer need so many calories. When I was a lay-person and interested in nutrition, I often found people who were overweight and older had decreased metabolic capacity (need fewer calories to maintain), so I would expect that in our monastic population if they are older and overweight already.