Thermal dynamics...
Smaller V6 will have less surfaces to conduct/radiate/convect/etc heat away
As Turtle says, it has the performance of a big block (amount of air/fuel), but
in a small V6
Heat rejection rates will be less, so the cooling systems must work harder
with higher flow rates (GPH, CFM, etc) both inside the engine and outside
so, when working that hard..the cooling systems lose ground and heats up everything
Computerized, so it is smart enough to dial back the amounts of power...shift
down to both save the tranny & engine.