GDP is all relative. Russia has an intact industrial base and can rebuild their military themselves and make good on loses. They didn’t foolishly offshore their means of production to make a quick buck....
The main problem with US military production is that our weapons systems are like exquisite Swiss watches. They work very well, but are terribly expensive and time consuming to produce, and not too much overlap with the civilian economy. Bush screwed the pooch by not replacing the inventory he used up, including ships and aircraft.