Sir,u have noticed very intetsting factor.
Actually in satellite,, EIRP: Effective Isotropic Radiated Power,
In simple lang, when we increses the value of eirp then the dish size will be decreases.
Why we increases or decerease the eirp: when the eirp will high then transponder will take more power to amplify & transmitt the signals with that it will increase the required power for operations but dish size willbe small.
If the kept eirp low then the dish size will be increases but the required operational power willbe low then it will gud for satellite because power/fuel is the life of satellite.
Btw, low eirp of gsat is not metter of worry because in ku we all have the 60cm dish & in c band it will not increase dish size too much.