There could easily be a level headed Nightkin, such as Marcus who appears at the front gate of Jacobstown, he could have started an experiment to see what the necessary steps where for the Behemoth to grow so huge, he could easily be occupied by the doctor who also occupies jacobstown, whilst they where into the higher stages of the radiation/behemoth test or whatever test.
The Nightkin from the Repconn test site could have found a huge storage of Stealth boy MK II's and travelled to Jacobstown as word of mouth was a problem that he mentioned, Neil could have told him about the place, so Marcus sees this amount of Stealth boys and overcomes to temptation and starts to use them constantly whilst still in his experiment which would then see it deteriorate from sane to insane to bat [censored] crazy.
During the experiment, the doctor tells Marcus (or level headed nightkin) and co that they shouldn't be dabbling in this sort of creation as it is playing god, Marcus being insane at this point, murders the doctor and Calamity and proceeds onwards into his experiment, clearly being new he gets frustrated and uses radiation in a high amount, the Behemoth reacts poorly to the amount used and escapes the sedations it would surely be under, it kills the Nightkin and therefore becomes Stealth, wandering the Wastes as a sort of legend.
Starting to sound better, but this isn't Marcus.
And Marcus isn't a Nightkin, he's a Pure/Prime Super Mutant.
And how would he know about Behemoths in the first place?
He's never heard of any East Coast super mutants.
And as
Joeyheadwounds said, each coast has a different viral strain in them.
Even if they (West coast SM's) wanted to they couldn't become any bigger than what they already are.
Unless they get a Super Mutant Edition Power Armor, but the only one that existed lies in the bottom of the sea with Frank Horrigans skeleton in it.
And if you suggest a different viral strain specifically designed to increase the mammals size then how would he know how to create this?
On a positive note, mammals do increase in size when dipped in FEV, correct?
Okay, imagine if the V87 super mutant dipped a Yao Guai in their vats. O_o
I think that could have worked.
But still, you'd need FEV for that.
And the only FEV left on the west coast is Mariposa Military Factory who is most likely under control by either Shi or NCR.
And West-Tek AKA Glow which most likely has some dried up FEV in the bottom of it's broken vats.