I'm not sure if the recent fledglings actually are larger or if it's more of an illusion.

Logically, it doesn't make sense that their bodies (sans plumage) would actually be substantially larger than the mature adult parents, other than maybe if the juveniles have a lot of "baby fat" or just different body shapes early on. I assume there are other aspects in play, like maybe they have more down feathers that push up on their outer plumage causing it to be more fluffed up and giving the appearance of them being larger than their parents.

Maybe a birdologology expert can weigh in (yes I know the actual term is ornithologist).