Their size isn't dependant on whether they are housed separately or together (except for the fact that housed together they will be stressed and therefore may not eat as much). There are also other contributing factors to size, one of which is gender. Another is not just how often they are fed but what size meals they are eating. I would suggest browsing the ball python forum and you will get answers to a lot of your questions that way. If you haven't already, you will read that rats are preferred over mice as a food item - are yours already eating rats?
0.1 Ball Python, 0.1 Creamsicle Cornsnakes, 1.0 Amelanistic Cornsnake, 1.0 Ghost Cornsnake, 1.0 Motel Amelanistic Cornsnake, 1.0 Okeetee Cornsnake, 0.1 Striped Amelanistic Cornsnake, 0.1 Silver Phase Miami Cornsnake, 0.1 Sunglow Cornsnake