OK WOW. That not so accurate jacket I have would easily fit someone with 18" arms.
Yeah, if the chest is say packed out 1" out at the top, you will want the base at least 4" deep.
Here's a look...
Note how even though just a Medium, it doesn't look too small? Length below the pant waist is just about right. It is tight in the shoulders, but good in the chest and waist. The Large size is 4" bigger in both chest and waist - too big.
20" upper arm.