that size doesn't sound too large for your ball, i would stretch out the time inbetween feedings if you plan to feed larger items (ie every 7 days instead of 5).
I believe some "know" when prey items are too big (even when they're really not): One of my balls won't touch anything larger than ~30 grams when she's almost at 600 grams and could easily take weaned/small rats. i think some snakes are just shy feeders when it comes to the size of prey they're offered.