Unless you uninstall packs, you aren't controlled the amount in a save file. It can make a difference for some people on really low-end computers because they just can't have the game doing that much at once. But not nearly as much difference as the number of mods/CC (and duplicates of them), the number of complex script mods adding systems, the number of Sims, the number of Sim relationships, how big the current household is, how crammed with interactable objects the current household is. For my saves, I'm pretty sure my biggest factor is just how many Sims I have + how many relationships, but I don't tend to put tons of stuff on lots.