I have the same problem as the OP on the other thread with my DTs, one eats loads and rarely seems full (doesn't seem to have the 'regulating gene'!)and the other does eat well, but often leaves food which her sister wants to finish for her.
I try to offer what I think is the 'right' potion size (e.g. 2oz of pasta each/ sandwich made from one slice of bread) based on how much they seem to eat, but does anyone know if there is official guidance on this? I know that each child's appetite is different, but I thought there might be something age-specific.