Design/Shaping, Aerodynamics, build quality, electronics and propulsion of the USA aircraft are all superior to the Russians.
Design/Shaping: Experience in stealth and the analysis of how radar reflects off curved surfaces gives the US an edge. This computer software developed from intense research and production stealth aircraft allow the US to have a large edge in stealth design.
Aerodyanmics: The US have a better understanding and better modelling software when it comes to Aerodynamics. This allows the US aircraft to make better/fewer compromises when it comes to aircraft aerodynamic design. Anyone can make an aircraft that can reach Mach 2.5, or perform 9G at 300knots, to do both is where its difficult.
Build quality: The US experience in alumnium and titanium manufacturer allow for a lighter stronger construction by using experience in load distribution. Decades of experience allow parts to be lighter without increasing the risk of failure. Tolerances of parts are far superior, partly due to the requirements of stealth. Reduced weight, gives increased speed, agility and range. All those one percenters start adding up to a large advantage.
Electronics: Western computer power has always been ahead of Russia due to large private investment. This allows for lighter more effecient systems, allowing for increased performance or reduced aircraft size.
Propulsion: The design and theory is more advanced so power to weight, level of compression and engine life are all superior in western designs.
Basically in summary aircraft like the 1980's produced SU-27 are using very similar technologies across the board and construction techniques similar to the 1960's produced F-4 phantoms. Aircraft like the 1970's produced Mig-25's are comparible to a scaled up 1950's produced F-104.