Absolutely you can connnect a smaller wire to the load lugs of a breaker.
Example 1: Start with a 100A protective device feeding #3CU @75°, go 300' and terminate them in a 200A breaker, leave the 200A with #3 conductors and go 20' to a 100A breaker.
Example 2: Start with a 200A protective device feeding 3/0, again go 300' and terminate in a 200A device, leave the 200A with #3 conductors and go 20' to a 100A breaker.
In my examples, the oversized breaker is acting as nothing more than a very expensive method to 'butt splice' the conductors together.
