Yes, all of my 3-month-old weigh that much.

The rule of thumb is, feed a prey item as big around as the WIDEST part of the snake. I feed my hatchlings every 5 days for at least the first year. Some feed every 7 days, which is fine as well. Do not feed less often than every 7 days.
At 9 months of age, I would expect a typical ball python to weigh between 300 and 500 grams. Some individuals are smaller and simply don't grow as quickly, though.
I would say--start switching the snake over to crawler-sized rats (just barely opened eyes). Feed large mice until the snake is switched. And try feeding more often.