Its all in the puddin'.
I had a Golden Barrel (Echinocactus grusonii) planted in my garden in California that, in 30 years, grew to the size of a basketball. I saw Golden Barrels in Hawaii at Kapiolani College that had been planted 30 years ago and were 3 feet across.
My honest opinion is that you do not have optimal growing conditions for a Golden Barrel or even really good conditions. It could be the size it is for a lot of years.
Daisy