Some do :-) Well, maybe not forever, but often until they reach the limit of the appropriate growing conditions. The sizes given are estimates based on average conditions, as with the sizes specified with any plant. Plants don't magically hit some predetermined size listed in books and immediately stop growing. They will continue to grow as long as they are alive, although once the genetically programed size is achieved, growth generally slows dramatically. And it is important to remember the growth characteristics of the specific plant in question - there are both running and clumping forms of OG's. The clumpers will most often follow that genetic programming and once they have reached the optimum spread, the increase will be so slow as to be perhaps unnoticeable except when comparing against several years. Runners or those spreading by rhizomatous root systems will continue to spread as long as conditions allow. The sizes listed for these are likely averages for the widest range of conditions - if the true potential was listed, many possible purchasers may be scared off :-)) It's similar to the convention of listing a 10 year size on trees and shrubs - on average, 10 years is about the maximum one deals with a specific plant in a specific landscape.