I can see how this makes sense i.e. getting the baby to do all the handling of the food/utensils means they can only eat as much as their coordination/dexterity allows. I don't think appetite levels and coordination necessarily correlate, you can have a baby that has a good appetite for solids but not so good with getting it in their mouth, or a baby who is easily able to get things in their mouth but has no interest in actually eating it. But it's a tricky one because how will we ever know if the spoon feed babies are bigger than they would normally be, or self fed babies are skinnier than they would normally be, or alternatively they were all exactly the same as they were genetically determined to be regardless of how the food got in there! There is no way of testing that.