Well as for size difference I find when fed on the same schedule "I feed on a 10 to 14 day schedule" some do tend to grow bugger than others and I believe it is genetic dispositions.
Here is a group photo to compare, The boas in the shoot are all 2011's except the El Salvador who is a 2010
The 10' El Salvador is barely bigger than the super hypo het anry sitting with him, She is getting huge in comparison to the others of the same age!
In the mix is a Sonoran, a Hog Island an El Salvador along with 3 morph boas from different breeders and litters
And in order of size with a comparison object included in the pics,
.
Sonoran
Hog Island
Paradox anry
Hypo het albino
Super hypo het anry
and finally the 2010 El Salvador
.
I hope this helps a little, As with people you will get variations with different parents and even variations with in the same clutch of babies from the same parents.