If muscles reduce when not getting enough nutrients and have memory, why do they not instantly regrow once a large new nutrient input is present?

It’s not the muscles that have memory, it’s our nervous system that innervates them

There is a positive feedback cycle with innervation sites at muscles (between nerves and muscles)….therefore, muscles need to be worked in order for the nerve to be there….otherwise, it retracts and the muscle atrophies

Nutrition is the next level of this process