I bred my serama roo to a frizzle hen..baby is half sized?

[Silverwarbler]

I bred my serama roo to a serama frizzle hen that was a bit smaller than him. I incubated 3 eggs but only had one hatch.
It's a little roo and he's extremely small. What would cause such a size difference?
Attached pic of son and dad.

 

[SilverBirds]

He's not fully grown yet. A few more months and he should be about the same size as his parents.