How about an experiment that went wrong and mutated a Death Claw to enormous size? Or the BoS where experimenting with something and new tech was fused with a super mutant or something lol? ya'know.
But who conducted the experiment?
Why did he do it?
For what reason?
Where did he get the funding, equipment and manpower?
What did he use on it?
Where did he operate?
What went wrong?
How did the Deathclaw mutate?
Was it over time?
Was it instant?
What chemical components did the doctor use?
I don't see how a Deathclaw could mutate into that size.
They are already products of the FEV, splashing some more on them wouldn't do too much.
So what other virus or mutagenic material could he use to increase their size?
Why would he try to make them bigger?
They are dangerous enough as they are.
Saying "oh well he's crazy" would be awful storytelling just to avoid a proper explanation.
How did he get it?
one man can't capture a deathclaw.
He needed mercenaries, at least a dozen of them, and he needed money to pay them for this suicide mission which has had to be pretty expensive seeing as how the mercs risk theirs lives trying to catch the most dangerous creature in the wastes.
Where did he operate and what kind of equipment did he have, how did he get this equipment?
I don't buy the "oh well he found an untouched bunker".
So he had to have been with a faction, why would this faction support his cause to make Deathclaws into Instakillclaws?
I see no way for this to happen.