Only problem is that it is a "smaller rocket" (less expensive) only if it is air-launched at altitude. Higher propulsive efficiency. Airplane fuel is cheap compared to rocket vehicle weight. This was the basis of the original Pegasus air-launched launch vehicle.
If you launch the same payload from the ground, the rocket could easily be twice the size of the air-launched version.
Only problem is that it is a "smaller rocket" (less expensive) only if it is air-launched at altitude. Higher propulsive efficiency. Airplane fuel is cheap compared to rocket vehicle weight. This was the basis of the original Pegasus air-launched launch vehicle.
If you launch the same payload from the ground, the rocket could easily be twice the size of the air-launched version.