Interesting question, I have wondered that myself. Could have been because that grip style was first used on the then-new .357 Magnum. I am not sure that is true but someone might know if it is.
The magna grips were not only thicker but also extended upward into the area of the web of the shooter's hand, providing much better comfort during recoil. A quick look at photos of S&W revolvers of the pre-WW2 era and the later revolvers with magna grips will clearly show this difference.
I don’t think anyone doubts that the Magna stocks were larger and that therefore the name is derived from Latin, Magnus = great, large.
What real nerds want to know is who determined that the new caliber would be grammatically neuter, Magnum, while the stocks would be feminine, Magna.
Of course, there are always two stock panels, so maybe Magna was intended to be read as the plural form of the adjective. But then it shouldn’t be Magnas with an s