It is a risk when you buy online. But like said, every individual of every litter is different. You can have very little one's, that seem like they don't grow. Others grow on air
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The most important thing is that you like him... You can ask for a little back, but don't get your hope's up and don't let it spoil the fun of the new snake.
On a site note: Snakes that are fed with mice grow most of the time faster...