As stated, the Load Calcs are what determined the Service Feeders' size and also what size of Transformer the PoCo installed.
The Main OCPD DOES NOT protect the Service Feeders, nor does it protect the Transformer. It only protects the Main Service's Bus Kit, and doubles as the Service Disconnect.
Most of the time, the Service Feeders are protected from being Overloaded by limiting the max KVA size of the Transformer.
I just finished up a Bank Branch T.I. where there are Two 1000 Amp services being fed from one 75 KVA [Z=3.0%] 4 wire Wye Transformer [120/208 VAC]. The Service Feeders were 500 MCM al and only one set per service [not paralleled conductors].
Even if the loads tried to reach 1000 amps, the max KVA of the Transformer would limit things by an abrupt Voltage drop when the Transformer became "Saturated".
If the case that the Tenant[s] needed to use 1000 amps continuously, by proper design the Transformer and feeders would need to be upgraded.