That is not quite correct on the Garand. Garand's rifle was originally made to handle the .30-'06 M1 ball cartridge. Pedersen (of Pedersen device fame, and Remington's chief designer) had developed a rifle that operated on a delayed blowback (unlocked breech) system. It could not be made to work with the powerful .30 ammunition, so he designed the .276 Pedersen* round, an intermediate cartridge, firing a 125 gr bullet at about 2600 fps.
The army board decided to adopt the .276 caliber as the service rifle cartridge and both Garand and Pedersen rifles were made up for that caliber for testing. The result was that the Garand design was selected as the better of the two and production approved.
So when MacArthur, well aware of the billions of rounds of .30 ammunition on hand, plus the inadvisability of the proposed retention of .30 for machineguns, thus adding a supply headache in case of war, cancelled the change of ammunition, it meant the end for the Pedersen design, where Garand simply reverted to his original .30 rifle, which was further tested and ultimately adopted.
The decision was not only good from the standpoint of economy, but the .30 was needed for penetration of vehicles and light armor, something the lighter .276 bullet would not have done as well.
*Not to be confused with the British .276 Enfield, a very large cartridge with a base over .52"; that was the cartridge the Pattern 1913 was made for and the reason the Pattern 1914 and U.S. Model 1917 have that huge magazine bulge.