Does the subpanel feeder need to match the main breaker size?
The feeder for the second panel, is sized to the load it's expected to serve.  The breaker in the main panel, that protects the feeder to the second panel.  Is sized to protect the feeder wire.
For example, if the load calculation shows that the second panel is expected to supply 60 amperes.  Then the feeder to the panel, only has to be sized for the 60 ampere load. You'll then need a breaker in the main panel, that's sized to protect the feeder.  So in the main panel, the feeder will have to be connected to a 60 ampere breaker.
tldr;
Feeders are sized to the load they'll serve.  Breakers are sized to protect the wiring connected to them.