Modern fuel injected cars have a catalytic converter that burns any fuel in the exhaust. Excess fuel in the exhaust and the convertor becomes a gasoline powered heater, turning it and all the pipe and muffler after it red hot. Our carbed bikes do not have this and don't tend to get really hot from excess fuel. Retarded timing can sometimes make the headpipe hot, but it does really tend to carry to the muffler.
Four strokes are much different than two strokes, much of the heat goes into the pipe and is conducted by that constant diameter pipe to the muffler. The pipe out of the four stroke cylinder will get so hot that sometimes it glows in the dark and the muffler will burn you or melt plastic. This is normal.
A two stroke makes the exhaust work. Right from when the exhaust leaves the two stroke cylinder it starts expanding. In those cones and fancy bulges make the exhaust gases work and expands more. As you know from physics class or even just spraying air with the air hose, expanding gases get cooler. By the time the2 stroke exhaust gets to the muffler, the energy and heat is much used up, the 2 stroke muffler doesn't get anywhere near as hot as a 4 stroke.
My guess is that you are just noticing this difference between your KX250f and your Blaster?