Don't underestimate the value of having what's intended to be a single craft in pieces that can be reused and combined with other parts. Having the Radish go from 1.25m at the top 1.875m at the bottom would work for me, given the scale and the fact that it should contain 2 kerbonauts. Having a separate 1.875m service module just means it can, with a generic 1.875m -> 1.25m or 2.5m adapter be used with other pods. Mixing and matching is one of the great strengths and joys of playing KSP.