I guess this refers to MiG-24 vs. F-15. Wikipedia writes:

The appearance of the MiG-25 sparked serious concern in the West and prompted dramatic increases in performance requirements for the McDonnell Douglas F-15 Eagle, then under development in the late 1960s. The capabilities of the MiG-25 were better understood by the West in 1976 when Soviet pilot Viktor Belenko defected in a MiG-25 to the United States via Japan. It turned out that the aircraft’s weight necessitated its large wings.

https://en.m.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-25