If I understand correctly from this question, The SSME's nozzle is made of pipes into which cool propellant circulate. The nozzle is larger at the end than near the combustion chamber. Thus, it must require either less pipes or smaller pipes near the combustion chamber than near the end (due to the bell-like shape).
How it this geometry handle? Are the pipes smaller? Are there less pipes? Is there other technique I can't imagine?