Drymarchon can easily get over 20 lbs if fed properly.
I have several males of different species all over the 20 lb mark.Most max out in the 15 lb range. Here is a shot of a Drymarchon m. rubidus male 5 years old @ 22 lbs taken a year ago. I have heard of specimens reaching the 30 lb range and 10 foot.

Alan B