This is what I liked about Hiddleston in THE NIGHT MANAGER, he never came across as the superman fighter, even when he gets out on top.
He never came across as any kind of fighter and that's why he's an implausible Bond. I had wondered if they'd left his military background vague because if they named something, such as the SAS, or even the Paras they feared the audience would say, "No way was he in the....." Or maybe that's how Le Carre wrote, i don't know!
But anyhow, Bond doesn't need his own fighting prowess to be diminished in order to create jeopardy - that just undermines the plausibility of the premise - the 00 section. The jeopardy comes from the poor odds for Bond to succeed and intimidating prowess of his adversary/s.
He certainly doesn't need to be a 'muscleman' in order to convince us he's a fighter, but he does need the moves and the physique of someone who practices those moves enough in his downtime to be as competent as he is on his missions.
Hiddleston didn't convince me at all in this regard in Night Manager. To an extent this is fine in context, since the character was no longer in the military. But even so, i'd see him as an ex-officer who pushed paper. However, if Hiddleston put in the hours toning and combat training i'm sure he could pull it off in the same way super skinny Adrien Brody did in Predators.
My favourite Bond fights are Connery & Shaw and Craig & Obanno in the stairwell in CR. Both are brutal and desperate and i'd find it difficult to believe that Hiddleston in his current extremely slender shape would last a minute in either.
Make no mistake - I loved Craig as Bond.
But apart from any "character-based" decisions - the truth is that short actors always want to bulk up because otherwise they just don´t look imposing enough.
Since most actors are on the short side...
As i said, it was his 'rationale', but as you say there could well have been other motives involved. I imagine the amount of stick he got when he chosen made him want to impress in every way possible.