Bloody hell this thread is a viper patch.
OP I had the same issue. I have a small appetite, my mum and sister and my ex have small appetites and it just didn't really occur to me that DS could have such a large one. Especially as he started off very picky when he was little, barely eating anything until he was two.
I was suffering with anxiety and I hated it when he said he was hungry because I'd feel that it was a criticism of me - that I wasn't providing good enough food and I'd feel desperate because I wouldn't know what to go to when he ate everything and was still hungry. But I wasn't really aware this was what I was feeling at the time. I just felt frustrated and confused. If I gave him bigger portions, he'd be overwhelmed and not finish them.
I used to be amazed at posts on here where people would say their DC ate multiple bowls of cereal, multiple pieces of fruit, more than two slices of toast, multiple yoghurts etc in one sitting because (I know this sounds strange) I hadn't really realised that was a thing people did. I would never have been allowed to do that as a child and as a parent, especially a single parent who struggled for money, it's so expensive to buy those things that I felt like I had to ration them out. The thought of seeing them disappear so fast was horrifying. Just to put some context here - I was seriously amazed aged 18 when I first encountered somebody buying an extra McDonald's burger to have on the side of their meal because it just didn't compute in my brain. I still find it impossible when DH will make himself beans on toast with four slices of bread and an entire can of beans. To me beans on toast is one or two slices and half a can. One day I had a huge panic over the fact that DH had used some mince to make us something for lunch because to me, mince is dinner food and you can't waste it on a lunch.
I now realise I have food issues - not huge ones but they are there - but I didn't really notice it at the time because it always seemed normal for me. (The mince lunch incident was a turning point.) I also read some things which assured me that a child's calorie intake can actually be greater than an adult's, because they are using the energy to grow and also because they tend to be much more active than adults.
I now follow DS's lead on food and he's allowed to have as much as he wants of normal food - that's things like sandwiches, salad veg, fruit, ham, cheese, bread, crackers, cereal, anything that's decently filling and not hugely expensive. Then he's limited on things like yoghurts, crisps, sweets, ice cream. Anything like pepperami, cereal bars, etc are saved for school lunches only. I try to make extra of main meals so he can have some more, or he's allowed to make himself toast with whatever he wants on it really - butter, jam, marmite. No limit on the toast. He has not got fat (I never really worried about "fat" but I think similar to you I wondered where all of this extra food would go). And yes, when he's wheedling that he's "hungry" he normally means that he wants sweets or crisps. He is a crisp fiend. So setting out the rules for snacks helped. When he says he wants something to eat in a whiny tone I now normally cut straight to asking directly what he wants because he normally has something in mind. If it's crisps etc then I think about how much junk he's had that day and say yes or no. And I also buy cheap snacky bits which are similar-ish which he likes but I don't have to limit as much. Ice pops last longer than sweets and are mostly water. Rice cakes are crisp-like (according to him) and cost about 30p. I don't do particularly protein heavy breakfasts because I need mornings to be low effort, he has cereal or toast or fruit and yoghurt, but he can have as much as he likes as long as there is time. I notice that it waxes and wanes, perhaps in line with growth spurts.
Some other points -
Going by both DS and some of my friends' children, it's fairly normal for 4 year olds to still be fussy or have a limited diet. Lots of them aren't any more, but for the ones who are seriously picky or stubborn about food, 5 or 6 seems to be the age that they suddenly get more adventurous.
Likewise, a four year old doesn't have to be a genius to figure out that asking for crisps for breakfast is worth a try even if they have always been told no. Four year olds are always asking for things they have never been allowed to do. It's what they do!