What is the yardstick to fix the maintenance for 2 BHK and 3 BHK?

Q: When we moved into an apartment, we paid the monthly maintenance based on the square feet of the flat to the builder. However, after forming the apartment residents' association, a few people are arguing that the maintenance charges should be equal for 2 BHK and 3 BHK flats. Can it be possible?

I understand that maintenance cost determines the number of square feet one occupies and Undivided Share (UDS) that each apartment possesses.

As a simple rule, 3 BHK will have an extra room and the UDS is more than the 2 BHK flat. So it should be based on the square feet each apartment has.

The 3 BHK flat owners argue that even they have equal number of bathrooms compared to 2 BHK. Here the question is not only the bathroom. The extra room consumes the electricity when the DG is On, they use resources like water to maintain the extra room. The 3 BHK owners also pay the taxes based on the number of square feet they own. When it comes to the maintenance payment to the society, why should it be different?

Please advise.

Thanks,
Madan