i don't want to come inbetween here but....
Off course proportion is it. But it still is better to have 18 inch arms then 15 inch arms, at least you already have that. Maybe in time you will go for proportion.
We may not forget (and i speak in general here) that no mather how hard we train like Dorian Yates, we will never look like him.
I mean training like Yates is not right for everybody.
In the old days everyone wanted to train like Arnold. You know, doing 20 sets or more for every muscle group. A lot of guys followed Arnold and where overtraining as well.
He will know when he overtrains. His arms will go down in size, maybe just 1/2 inch, but he will know.
Also not everybody is in the thing to train every muscle group in the same intensive way. The most important thing is to train and do what you want to do or you will give it up.
I am not saying that training arms 3 times a week is the right way, you read the answers. I also think every 4-5 days or for some every 5-6 days is great.