This exploded myth I see often in gun magazines and is quoted by gun writers.
The 44 mag cartridge can handle more pressure because the case walls are thicker.
Go measure them. You will find that the 45 LColt has thicker walls than the 44 mag. The currently produced 45 LColt cases are being made thicker than the originals. The originals were only .003 thinner than the .44 Mag. cases. Which is hardly a big difference in thickness. But what really makes the .44 mag handle more pressure is the fact that the case is smaller .457 dia instead of .476 dia. So the cylinder walls have more steel left in them after they are bored out. So there is more safety margin which means you can bump up the pressure of the cartridge safely. More pressure means more velocity and better performance.
Except for some reason a big slow moving bullet doesn't seem to care if it is not a magnum.