It's not just more propellant. Note the engineering that went into keeping the thing from burning up. Or look at the MESOS rocket--the guy who built that took

It's not just more propellant. Note the engineering that went into keeping the thing from burning up. Or look at the MESOS rocket--the guy who built that took the choice of delayed ignition of the second stage, accepting the extra gravity loss in exchange for less drag loss. And if you don't have a guidance system you have to complete your burn low enough that aerodynamic forces keep your rocket pointed up during the burn. The bigger your burn the more of an issue that becomes.