You can check it taking an example also. I'll explain why it happens so.
Actually we can say Round Robin is nothing but FCFS with preemption. Actually the TIME QUANTUM used in the algorithm, introduces preemption and hence makes it better than FCFS in terms of response time.
Now, if my time quantum is as large as the largest execution in the set of proccesses then, even the largest execution time process will not get preempted. In a nutshell, I can say that we removed preemption from the algorithm, hence it is just left with FCFS, and hence it degenerates into FCFS.