That's a different kind of muscle memory(although there might be some overlap in the exact case you're speaking), which is actually far more akin the common association of the word memory.
'Muscle Memory' as far as growing back to a size, regaining strength at a rapid pace, has a lot to do(as Brick's abstract notes) with reparation of neurological pathways after a long period of atrophy which lead to denervation. Neurological adaptation is huge when it comes to strength, and the ability to quicken the regeneration of those pathways leads to a very rapid rebound in ability to recruit muscle fibers, which is very key in their growth. The word memory in this sense is a slight misnomer(not a complete one, it's just most people associate memory with brain activity, recall/recognition, the brains process of encoding).
That's a really interesting road to go down. I've always heard muscle memory referenced in response to periods of serious muscle atrophy(for the myriad reasons it may occur), or as an effect of previously experienced supraphysiological conditions(steroid use) and the bodies ability to then 'remember' and retain some of the previous condition even though (hormone levels in this case) have returned to normal. Thinking of it that way, it sounds entirely plausible that as long as some beneficial neural adaptation has taken place while the body is heavier, that a future leaner version of the same person should be able to return to that state again(or at least approach it easier, as with the steroid example returning exactly to that state or maintaining it will be impossible because physiological conditions have changed and limit it in some form).
Sorry for semi-walls of text. Also, I'm not a PhD in the field being discussed, just someone who reads research for self education and (nerd alert) fun.