Aren't halfling supposed to have a disproportionately large head though?
No, in fact - that's never actually been any part of their description, at any point in the game's history.
The disproportionate bobble head is only one artist's depiction, and even within the 5e players handbook there exist several other far more realistic and believable depictions - it's just an unfortunate coincidence that that particular one was the one that ended up on the halfling race page.
There's also no actual danger of mistaking a halfling for a young human, in reality; young humans, unless they are literal 1-year-old infants, are actually taller than your average halfling, and have different proportions, to the extent that anyone living in a world with halflings in it as a people would not be incline or likely to make that mistake.
Just so as not to derail from Black Elk's thread here, however - There's a major thread that discusses halflings, how they are depicted and represented, and goes into detail about the models being used, and you can find it here:
Halfling Feedback.
