My 06 okeetee female born in June, so about 15 months old currently, is 271g. 06 Blizzard female a month or so younger is 265g. Both are a little large for their age, which might be due to a diet of mostly African soft furred rats. They have good muscle tone and are not fat. I feed them on a schedule similar to the Munson plan that I came up with on my own over time. I also slit the backs of all prey which probably helped them grow a lot faster. I imagine growing my own feeders helps as well, since I can ensure they are all well fed and healthy.
Corns grow at different rates so you can't really make a blanket statement that they are "too big" for their age.