Callouses are caused by rubbing. There are a couple of things that could cause them in that spot.
I've got a couple of questions:
Did you always have somewhat of a callous there, and its just lately gotten bigger? Or is it newly developed and getting big all in one? A callous growing can be caused by a change...did you change your span, or change how you grip the ball, or change the amount of games you bowl a week drastically?
Before the callous developed or grew, did you already have to apply much force to get your fingers in the grips? Fingers should slide into a grip easily, touching all around on the grip, but not needing to be forced in. When I insert my fingers in the ball they initially touch all around, but once I put my thumb in and my hand stretches a little bit, the only part of my finger that touches the back of my grip is the end of my fingernail. The back of my fingers dont touch.
It's also possible that your span is a little on the short side, so you grip with more finger pressure, really trying to dig into the ball, and end up with the back of your fingers rubbing there.
My best guess is your grips were originally too small, such that you could in them, but you really had to push your fingers in, and they rubbed there throughout your release, and now that youre getting a callous you can't fit your finger in the grip at all no matter how hard your press. It may feel different, but when you do get your grips changed, make sure you can fit your finger in without having to apply force.
