Mass does not strictly matter, it is the ground TWR of the launch config. Heavier rockets do tend to have a lower TWR but it is not a universal rule.

Starliner is also held for confirmation of all systems being functional even with the SRBs. Better to let them burn out on the pad and clean up the mess than to risk release with, say, asymmetrical thrust. That said I'm sure there are variances between the two rockets' procedures which could make one appear faster.