the proportions of the CEV will likely be somewhat different that that picture shows. Recent articles suggest that they're looking at using a 5 segment SRB instead of the current 4 segment design. The proposed CEV heavy lifter is designed around the 5 segment SRB. Imagine putting the 2nd stage on top of the taller SRB. The second stage will probably be somewhat smaller than shown since the bigger SRB will reduce the amount of propellant needed to achieve orbit.

Given the problems they're having converting the SSME to air-start and insufficient performance of the standard J-2 (and perhaps even an upgraded J-2S) engine, it seems like a reasonable tradeoff to me.