Can I move the main breaker box to inside a small bathroom?
According to National Electrical Code,  overcurrent protection cannot be in bathrooms. So no, you cannot put the panel in the bathroom.

National Electrical Code 2014
Chapter 2 Wiring and Protection
Article 240 Overcurrent Protection
240.24 Location in or on Premises.
(E) Not Located in Bathrooms. In dwelling units, dormitories, and guest rooms or guest suites, overcurrent devices, other than supplementary overcurrent protection, shall not be located in bathrooms.