Chen Xi was, in fact, speaking truthfully; with the current potential under Liu Bei's rule, along with the combat power that can emanate from the existing foundation, it solidly surpassed Parthia by a head, and it was the Parthia that Vologases V had just about completely integrated.
Of course, the overall combat potential still falls short of Kushan by a degree; there's no way around this. If Liu Bei could swallow Sunx Ce, and Chen Xi completed the integration of industries under Liu Bei and Sunx Ce's rule, the combined might would basically be able to steadily surpass the Kushan controlled by Vasu Deva I.
Of course, part of the issue lies with Kushan's perpetual inability to completely integrate the industries and combat power of their nation. Internal religious frictions and conflicts in their training systems inevitably lead to some internal consumption of Kushan's strength.
