By girth at his midsection, i hope you mean at it's widest point - this would be about 2/3 the way down from the head, which also, not coincidentally, is where his food will sit. I have never seen a hatchling ball python with a girth as small as you describe. I am not saying you are mistaken, only that I haven't seen it. Adult mice are a starting point for your python, not an ending point - These guys are meant to be 4.5-6.5 feet long, and thick as a softball through the midsection- if you feed them mice all the way, they'll be consuming 5 or more a week - that's a lot to spend, and a lot of time..... one medium rat weighs the same as 10 adult mice (smallish adult mice, what you normally get when you buy bulk frozen). The ratio of weight to volume gets better (or worse, if you feed mice) as you increase to a large rat. Adult bp's are more than capable of eating large rats. fuzzy mice are an appetizer, and a scant one at that (think one piece of popcorn chicken!).
Anyway, best of luck to you. All of the above are simply a reflection of my opinions and experiences. As all the mutual funds are quick to say, individual results may vary.
Regards,
Neil