FSS vs. BSS (Or DBS)
FSS is not allowed to have as much power as BSS (Broadcast Satellite Service) also generically called DBS in this forum. The power is measured within the footprint in decibel watts (dbW) at the receive antenna (dish). It is a function of downlink transmitted power & antenna gain, and footprint size and other less important factors. The fact of the matter the allowed dbW by FCC regulations is less for FSS than BSS so one can conclude for equal fade resistance, FSS requires a larger dish.
Everything else being equal, FSS has SLIGHTLY less gain than BSS at the antennas because higher frequencies produce more gain. I'll post the formula if someone is interested. The difference is not that significant in this case as both services are in the Ku band.
And yes as was already pointed out, the larger dish has more immunity to adjacent satellite interference. Other factors include off set dishes being more immune than prime focus dishes and the focal length to dish diameter ratio. FSS has 2 degree spacing while BSS has 9 degree spacing and that accounts for the FCC permitting higher power (dbW) for BSS. Interesting, the 9 degree spacing was based on analog vice digital service and since digital service is more resistant adjacent satellite interference, there's a good possibility that BSS may some day be allowed 4.5 degree spacing.