The record is hold by Voyager:
The programmers must have done an outstanding
job, considering the slow processor and limited memory. At launch,
only two words of free space remained in the 4K of plated wire.
Only two words of free space in a 4K memory, this record is unbeatable.
Source: Computers in Spaceflight, p. 178
If I am reading wikipedia right the honor goes to explorer 1 with zero memory.
Acording to the article the tape recorder was not ready so the science data was transmitted in real time.
I think that despite the lack of enough empirical data there are some basic assumptions we can agree on.
Since astronauts can adapt well enough, even for 1 year of living in zero G, full 1G Earth gravity likely isn't necessary for permanent living.
A person with a healthy body mass index (BMI) shouldn't have a problem to adapt for living with 10% lower or ...