It all depend what area you defend or attack.
MIG-31 for Switzerland might be a bad idea, But for Russia you can not find better.
And yes size do matter, bigger radar dishes, stronger power output, stronger processor, more fuel and so on.
Thats why I find it funny when F-18C radar react on Chaff in one video but some believe aim-120 would not, those who believe that dont believe in Physics.