Quantity was only one factor in the arms race. There were also questions about different systems giving you different options (e.g., early rockets were inaccurate and so needed large warheads and could only target cities; later rockets were highly-accurate and so could use smaller-yield warheads and be used to target harder, smaller targets like airfields and missile silos), about vulnerabilities in existing systems, about countering developments the other side made that seemed to endanger security, and so on. This is separate from the dynamics of the military-industrial complex (which acted in somewhat similar ways in both the US and USSR) that made money and gained influence from developing new weapons.