100% agree seeing a doctor is the best idea if someone is not losing weight despite their best efforts.
But "eating almost no calories" can also cause the body to retain fat and there are other concerns that sometimes need to be addressed.
This is said a lot, and I did believe it for a long time. However, it isn't really true. It actually doesn't make sense if you think about it without preconceived notions based on common advice.
Also, I'm gonna possibly sound condescending below because I'm using a kind of silly way of explaining things, but I'm not trying to be condescending. This (imagining the body talking) is just literally how I think of it.
Basically, we all know the body saves up fat in preparation for future deprivation. Our bodies are like, "I better hold on to this fat in case we wind up starving in the future. If we get into a situation where we are starving, this fat will be so useful!"
Fat is stored specifically because we might need it later.
So when we impose deprivation upon ourselves through calorie restriction, the fat is not held on to. Instead this is what the body would say:
"Whoa! Glad I saved up this fat! We need it for energy now. Let's burn that fat!"
It just wouldn't make any sense for the body to be like, "We've been saving up fat for the specific purpose of using it if we experience a period of deprivation... but... nah. We won't use it after all."
The natural lowering of metabolism when one loses weight is not actually a bug. It's a feature. The metabolism is not getting damaged. It's just that the lower your weight, the less fuel you need. A lot like a small car vs. a big car. The small car is not "damaged" since it needs less gas. It's actually more efficient. You have a lower TDEE (i.e. amount of calories you can eat without gaining weight) when you are smaller because your smaller body can sustain itself without as much energy. It's more efficient!
Additionally, the difference in TDEE between people who've lost a lot of weight and same-sized people who were always small is mainly tied to NEAT - non-exercise activity thermogenesis. The people who were always small are shown in 24/7 type controlled studies to be more fidgety, more likely to pace around, etc. They just... wiggle and fidget more. lol. The person who lost weight and burns fewer calories than their always-small cohorts will have the same metabolism of their always small cohort so if they just go for a twenty minute walk every day.
With all that said...
Yes, water retention really can make someone larger even if they don't have a high body fat percentage.
It's also true that certain sorts of foods can make somebody hungrier. Sugar makes me very hungry. It was not really the sugar that made me fat - it was the way sugar made me want to eat more in addition to the sugar.
And a lot of illnesses or medications make somebody so hungry that resisting food is almost painful. Even something as simple as ADHD can do it. People with ADHD who start stimulants don't actually lose weight by magic. They lose weight because
Their hunger cues weaken and so they take in fewer calories
They usually engage in more NEAT (non-exercise activity thermogenesis)
They have better self-control in their medication
It's all very interesting