To answer the question-is is possible to have 20 inch natural arms, you would have to qualifiy that with some parameters-ie, fat content, etc and that has already been mentioned. I weigh 240lbs and have 20 inch arms-cold before a workout-I have posted some links to pics in the past and you could see that I am in decent shape, but certainly not bodybuilding shape. Most people tell me I look like a well built pro-wrestler, but if you put me next to an amateur bodybuilder, or even a well built gymnast, I would look fat. I would venture to say that havin 20 inch arms with "bodybuilders" body, though is possible, is not very likely-unless they weigh well more than 250lbs-and there are a lot of those types of guys out there. When I started lifting, they used to talk about pro bb'ers who were 220+ as being "monsters" and now that is considered a lighter weight.
In addition, if you ever see a guy at the gym or out in public, who is wearing a short sleeve shirt, etc and the first thing you notice is that the guy has huge arms, chances are, he has little development elsewhere-minimal at best-if you see a really well built guy, who is proportioned correctly, you dont realize how big he is until you get up close to them-the symmetry sort of overshadows the individual muscles. Just like if you ever glance over a a female and you sort of go "wow" check her out-chances are she has 2-3 features that really "stand out", but the more you look, the less "wow" appeal she has. The most incredibly gorgeous women that I have ever seen all had one thing in common-the more you looked-the more you appreciated their beauty, but there was not one thing that really stood out.
ok, I am rambling now, but I am home with the babies and my wife has to travel out of town because her best friend's father was killed two nights ago and she is attending the funeral. I took a day off of the clinic and the babies are napping and I am enjoying the solitude.
and...Maki, if you read this-outstanding GPP article, great, great stuff