Yeah, it's a lot of wealth, and a lot of power. Coupled with a man as charismatic as the Kingmaker, and it's a very dangerous brew. Norfolk is probably best of the rest behind the Beauchamp-Neville-Despenser behemoth and Lancaster, you're right.
Yeah, there's a lot of imponderables, but dividing the land was much safer. If Edward had given more land to the male line inheritor (George Neville, Duke of Bedford, Montague's son), it may have helped even more.
The ideal for the crown is that this Neville ends up as one of the near 25% of nobles who grows to see himself unable to produce male heirs, and marries his daughter back into the royal family, thus returning the land to the crown. That needs quite a bit of luck, though. Timings of births have to vaguely fit, and no foreign alliances needing cemented by matrimony.