I’m no expert, but if memory serves the Flanker is a beefy boy. I suppose it has a bit more room for juice and engines than the Mig 29.
The MiG-29 and Su-27 were designed as complementary aircraft based on the same set of aerodynamics research by TsAGI, the Soviet equivalent to the aeronautics part of NASA. The 29 is the cheaper, shorter-range point defense fighter, and the 27 is the larger, heavier, and longer-range counterpart.