Why is it a problem? My BCC is over 3 years old and only just now 5 ft. Thats slightly less than half her potential. She's being slow grown the right way and is very muscular.
Boas are different from balls. If you pose this question on a boa specific forum, you would get the same reply from many more people..
People always seem worried and try to feed their boas to get them huge. The truth is, they should grow much slower than you'd think. The over fed ones that become too big, too fast, don't live as long, and don't produce as well if bred. Fact..
Your boa is fine. You'll notice a growth spurt before long and the difference will be mind blowing. Trust me. It's how they work.