It's the eyes.
As you can see from this picture, the lavender has a normal-sized eye for a hatchling. I don't have good enough resolution on my ruler to give an exact size, but it's visibly over 2mm in diameter.
The butter (and siblings), which are larger hatchlings, have smaller eyes, 2mm or less in diameter. The overall effect causes them to appear to have longer heads.
I have no idea if this defect is something worth worrying about, or if they will grow out of it. The fact that nobody here noticed the odd eye size makes me wonder if I need be concerned at all. Thoughts?
-Kat