It's not 'bulking agents' !
The difference is that the protein in 'hungry baby' milk is not 'modified' - it's the same as ordinary cows milk. It's casein-dominant, in tech-speak.
The 'non-hungry baby' milk is 'whey-dominant' - some of the casein in it is taken away.
Casein is tougher for babies to digest, and so the milk stays in the stomach for longer (supposedly - there is no research on this that I have ever seen) and the baby is 'satisfied' for longer (again, no research).
But the calorie content is the same and both types of milk are 'approved' as breastmilk sustitutes by the department of health for use from birth.
The idea that it is a terrible thing, or bad for the kidneys, or whatever is, I suspect, manufacturer-led. Obviously, both 'non-hungry' and 'hungry' are far closer to each other than either of them is to breastmilk, anyway. The way these types are marketed is deliberately uninformative - where do you ever see anything about the casein-whey-modified-no-modified thing I have outlined above?
Manufacturers want there to be different types of milk - note, all brands have these different types. That way, consumers can start on one, then move to another, and then move to follow on and then move to toddler milk. Segmenting the market in this way encourages brand loyalty, gives the illusion of carefully-matching the baby's needs to the milk, and allows for a greater range in which to carve out a brand.
There is no risk that 'hungry baby'milk would be any more likely than the non-hungry milk to make a baby fat - how could it? The calorie content is the same.