A 40g is too big for the largest of ball pythons. A 20g is too large for a hatchling. A 10g will last you until the snake is around 600 grams.

Your ball python may be "curious" but ball pythons are not active animals. They spend 90% of their time hiding. They don't "enjoy" extra room. People enjoy extra room.

I would also go with the fan idea. The high humidity is because of lack of airflow. Just stick a fan by the tub and that should help you out.