In LSM, balanced design of RCC beam gives smallest concrete section and maximum area of reinforcement.

IS code does not permit over reinforced sections, so balanced section will have maximum A_st (A_{st =} A_stLim) and to resist same bending moment by under reinforced section and balanced section, Lever arm has to be increased in under reinforced section as tension r/f is less and hence cross-section dimensions has to be increased.

Therefore balanced section will give smallest cross section.