In order to improve the efficiency of quantum secret sharing, quantum ramp secret sharing schemes were proposed (Ogawa et al., Phys. Rev. A 72, 032318 []), which had a trade-off between security and coding efficiency. In quantum ramp secret sharing, partial information about the secret is allowed to leak to a set of participants, called an intermediate set, which cannot fully reconstruct the secret. This paper revisits the size of a share in the quantum ramp secret scheme based on a relation between the quantum operations and the coherent information. We also propose an optimal quantum ramp secret sharing scheme. [ABSTRACT FROM AUTHOR]