Well he looks like he has a couple meal in his midesection in the pic but the lower half, where fat is collected, is very thin. The neck size (or small head as you put it) may have something to do with it being a cross. The neck size has nothing to to do with it being fat.
IMO after feeding snakes all they can eat and then properly cylcling them (not forced cooling) and then allowing males to breed and females to get gravid.. Tt is impossible to get a snake "fat" unless it has a problem with breeding or it has a disposition to collect fat cells-which is rare.
It is even more impossible to get a kingsnake (notice i said kingsnake and not boas or pythons) FAT, while it is growing to a mature length. Once it reaches maturity if the KEEPER allows the snake to go through its normal cycles you should feed the snake as much as it will eat.
...If the snake is a "pet" then different rules apply and an adult kingsnake can get obese.
If you got a fat snake then BREED IT! That will sure as heck keep it thin.
I have a large colony of Florida kingsnakes that i have been maitaining for several years and i feed all my snakes as much as they will eat. None of them are fat except one which has a defective genetic disorder.
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