Hello. I agree activity in the day is pretty unusual for a ball python.. and as you said it probably is just settling in… However, why is it necessary to decrease the amount of space it is offered? In my experience the more room the snake has to explore the better?.. As long as there are lots of hides (at least 3) and the snake can get from one side of the tank to the other without being seen to make sure it feels secure… what is wrong with a bit of extra space to stretch out and roam around at night if the snake decides it wants to??
Young BPs usually do better in smaller confines as it decreases the amount of open and overwhelming space, these animals spend most of their lives underground and generally only only leave their burrows to hunt, and then find a new burrow, and mate. Larger habitats with many hides can work out but they sometimes do not promote a good environment to settle in and stress behaviours may continue to persist, the hatchling will continually change its spot because it feels exposed and not claim an actual “burrow”.
And at two months old, I also really wouldn’t expect a hatchling to be large enough (likely well under 200grams still) where it couldn’t comfortably stretch out in a 15qt tub.