/u/restricteddata ably covered a goodly portion of this question in his answers to Why did the United States develop so many different fighter aircraft during the early Cold War? and How did post-WWII Soviet fighter aircraft designs differ from those of the US? How much of the difference is attributable to military doctrine? To economic necessity/. One thing both of these answers do particularly well is that they highlight that quality/quantity is not as a clear-cut dichotomy as some popular commentators on military subjects maintain.

A big difference was that the US favored quality over quantity, and The USSR vice versa (not to say they didn't want the best aircraft ). The mig 21 was built in massive numbers as were other lightweight fighters. The U.S. Used advanced tech, like the F14 or F15. They were and are more expensive to produce. The F16 was a response to the massive numbers of Mig 21s being built in the Late 60s and the need to match at least most of them. The F5 was also designed so that the U.S. Could export them to smaller nations like the soviets were with migs

Most Soviet aircraft were designed to fulfill a singular role, whether air superiority, attack, close support of ground troops etc. So generally they could do one thing and one thing only. A good example would be the MIG 25, which was designed as a high altitude, high speed interceptor. It carried only missiles, and was meant to be able to catch fast and high flying reconnaissance jets, bombers and other planes that normally operate at high speed or altitude. But it couldn't dogfight, perform ground attack or anything else really. Ironically this aircraft inspired the development of the F15 Eagle, as the USAF believed it was a new Soviet superfighter. The Eagle, on the other hand, is equally effective at both air combat and ground attack, with the Strike Eagle being based off the 2 seat fighter version and featuring enhanced ground attack capabilities. Even in the 60's, the F4 Phantom was a capable fighter bomber as well as one of the world's premier air superiority fighters. The MIG 21 however, did not have an effective ground attack ability due to its low payload capacity. Soviets never had a true dual use aircraft until the advent of the excellent Sukhoi 27, and that was towards the very end of the Soviet Union.