My health visitor (who is far from infallible, so perhaps wrong!) told me that it's that a 'portion' of each food is their palm, not the whole meal - so an entire 'meal' would be, say, one palm of egg, one of toast, one of banana and one of avocado (one protein, one carb, two fruit/veg). DS sometimes manages that, more often doesn't. We certainly have to offer him around four times that to get that amount eaten, because so much ends up on the floor, smeared across the tray, etc. (does depend on the food, though - most fruit gets eaten quickly, efficiently and with little waste; things he's not so sure on, like egg, have a lower offered:eaten ratio!).
There has been a sudden and very noticeable increase in the amount he's actually eating in the last week (he's eight months) - but we saw a massive slowdown (in fact, a near total food strike) for the two weeks before that when he was getting his top teeth - which drove me demented with worry, but ended as quickly as it started.
I worry a lot about how much DS eats - but, actually, that makes me sort of glad he just won't be spoonfed because I suspect that, to my shame, I might be a 'just one more mouthful' force-feeder if I was feeding him, and if I could track exactly what he was eating, and that's really not good for them and their long-term relationship with food. This forces me to put him in charge of what he eats - and so I try and trust him to know what he wants. It is hard, though!