Manufacturers during the black powder era used the adjective Nitro Express to denote a more powerful round. This term fell out of use with the advent of smokeless powder. In the late 1930s Smith and Wesson coined the adjective Magnum to denote a more powerful round.

The 35 Whelen began life as a wildcat in an attempt to increase the range and power of the 30'06 round. It was developed in 1922 over a decade before the term Magnum was first used. As a wildcat round, it is fireformed from necked up 30'06 cases. This round is one of the few wildcats to survive WWII. For a long time it was the one of the few hard hitting non-belted cartridges between 30'06 and the hard kicking 416 Rigsby. Perhaps this is one of the reasons it has survived to this day.

Comparing a 30'06 load
150 gr IMR 4895 48.5 gr. 2,731 fps
with a 35 Whelen load of similar velocity
200 gr IMR 4895 61.0 gr. 2,791fps
The 35 Whelen load fills the case with 25% more powder and pushes a 33% heavier bullet to a similar muzzle velocity.

Comparing another 30'06 load
180 gr IMR 4895 43.5 gr. 2,520 fps
with a 35 Whelen load of similar velocity
250 gr IMR 4895 55.2 gr. 2,511 fps
The 35 Whelen load fills the case with 27% more powder while pushing a 39% heavier bullet to a similar velocity.

Now finally compare the muzzle velocities of a 200 grain bullet in 30'06 Springfield, 300 Winchester Magnum, 350 Remington Magnum, and 35 Whelen loads and you will see why the 35 Whelen cartridge has earned the nickname "the poor man's magnum".

First a 30'06 load
200 gr IMR 4895 43.0 gr. 2,445 fps
and a 300 Win Mag load
200 grain IMR 4895 58.0 gr. 2,631 fps
then the 350 Rem Mag
200 grain IMR 4895 60.0 gr. 2.732 fps
finally the 35 Whelen load
200 grain IMR 4895 61.0 gr. 2,791 fps
The 35 Whelen uses 42% more powder than the 30'06 and pushes the same weight of bullet some 14% faster. Not bad for a cartridge that can be made cheaply by necking up inexpensive military surplus brass.

The NRA did studies and found one of the reasons the .308 Remington is more accurate than 30'06 Springfield is that the .308 case is more completely filled with powder. The air bubble in the 30'06 can cause different burns depending on where it is when fired. For instance, is it near the primer or the bullet or along the side?  So how does the accuracy of the more completely filled 30'06 cases of the 35 Whelen compare to the .308 necked up to 35 caliber rounds of the 358 Winchester? Turns out their accuracy is similar.

Note  the volume of a .358 bore is 35% larger than the volume of a .308 bore of the same length barrel. This larger volume equates to lower pressure everything else held constant. Pressure is important to the gas system of an M1 Garand that's been rechambered in 35 Whelen.