Muscle memory, actually is mostly water. Its not an actual integer that says you had x grams of fibre before so quickly rebuild it.
When you train, you increase the nuclei in the muscle. It simply means you concentrate more cells. Resistance offers the most dense build, where as hypertrophy offers more volume. All these cells are filled with concentrated substance, and via the osmotic principle, with water.
When you stop training, you stop the flow of nutrients going into the musclefibre, the cells will significantly lower in nutrientconcentration, osmotic pressure will drop and you cant retain as much water in the cells.
"muscle memory" actually refers to these cells. When you start training, you create muscle pump (time bloodvessles open), you get nutrients back into the cells, and more water can retain itself into the cells.
So what you've basically done, is filled out your muscle fibers again. Wich happens before any form of increased lean mass takes place
Let me give you an example of just how much effect this gives
I've lost 12kg in about 10 days when very high antibiotic usage crashed my liver enzymes and as a consequence my water retention in the muscle, and i was on 500-800kcal due to stomache issues.
I've lost 3kg of fatfree mass it appeared afterwards, 9kg was simply water. And i continiously train. The reason i lost these 9 kg of water was because i was barely producing any musce glycogene, wich is about the most important nutrient in the cell considering osmotic pressure.
When my liverenzymes restored themselves to normal activity, within a month i've got the missing 9 kg back.
Same story can be easely seen when using water retaining steroids, and non water retaining extremely potent Trenbolone.
High short estered water retaining steroids, give a very fast increase in mass, wich is mostly down to the increased water that can be stored. You'll bloit quickly. After a cycle, you'll also lost quite a bit.
Trenbolone doesnt retain water. It has a halflife of 8 days(enathate)/3days(acetate). You will not feel any strength or mass increase from Tren whatsoever, before the first 2(acetate)/3(enathate) weeks. This is actual lean mass gain.
And 500/500 anabolic/androgenic ratio Trenbolone, is MUCH more potent then that tiny bit of unestered testosterone we produce naturally.
But keep up the good work, you'll notice a drop in mass gain. What you gain after that, is actual muscle.