Yes, all sims outside the active household gain/lose weight and muscle. To stop that you need either MCCC (need to set no body changes for each sim individually) or aforementioned RoBurky Fitness controls (works globally).
There is a specific case I ran in when using both mods, it's probably rare, but I mention it anyway: If you set sims outside the active household to not change body shape while unpayed, and certain sim(s) within your active household to also not change body shape even when played, then Fitness Controls needs to load before MCCC (rename the files to start with a letter lower than M).
