She's smaller than everyone else because of whoever had her before you.
If it makes you feel any better when I got my first snake she was 24g at 8 months old.
She's now 224g, (at 17 months) of course she's having feeding strike issues right now but she's grown in leaps in bounds, so don't worry too much as long as your snake looks healthy, doesn't look all spiney you should be fine.