How does a large game save develop?
Does it come from managing the household with the collection?
Or from building houses in the worlds, as well as decoration existing houses?
Is this also dependent on the purchased packs and kits?
HakhaKZ1 Saves get larger the more sims you've played and the longer you've played them, and the more houses you've built and the more complicated those builds are. Saves can also get larger due to various game bugs. And the total number of sims, including unplayed ones, can drive up the size as well, so make sure the game is allowed to cull unplayed sims.
If you want smaller saves, create a different save for each household you want to play; don't just drop them all in the same save. And don't place houses or community lots you're not going to use. It's fine to have a few interesting builds, just not several worlds full of them.
You can also check your save file size every time you quit, and if it's getting larger, think about what changes you made in the last play session.
There's no magical formula to keeping saves small though, other than not playing the game much at all.