Co.op.hsg.society mumbai, maharashtra

We have 13 Flats in Co Op Hsg Society Mumbai at Vile Parle West having Built up Area as under: 5 Flats having 830 Sq.Ft. 5 Flats having 970 Sq.Ft. 1 Flat having 1124 Sq.Ft. 1 Flat having 1830 Sq.Ft. 1 Flat having 2910 Sq.Ft. The Total maintenance charges for smaller Area Flat is ₹13/- per sq ft and for largest Area ₹ 5/-per sq ft. This is very unethical. How to resolve this under Model by laws or any Other prevailing Laws or Court Rulings.