Hi and welcome!
Can you give us a link to your pattern?
It may well be that the difference is in the tension that you're getting with your yarn and needles. If it's smaller than the given tension, that'll certainly make the cardigan back smaller.
The Faroe super chunky knits at 9sts/4" so 47sts should be about 21" across the back although I'm not sure what your pattern stitch is. What are you getting as a measurement across the back?