It's because GT is more realistic.
For some reason, on Forza Motorsport, stock M4s, Supras, even Golfs spit flames with the stock exhaust - and it's a repetitive animations with no deformation, no difference between one flame and the next one.
On GT7, it's actually the most detailed flame-spitting from all racing games I could find. They vary according to RPM, Turbo level and pressure, and the exhaust kit. Some cars with lower level exhaust and turbo kits might spit a smaller orange flame. Full on tuned or race cars will spit a bigger blue/purple looking flame. And it all varies with how fast you are going. Just much more detailed.