Just an update from my play from last night (which is now 9ish pages ago) regarding normal game play and file size increase. That is, how much the game seems to grow through normal play and why (theory). Which doesn't leave much room for people to 100% complete, if at all (depending on bloat from other factors such as crafting, dismantling and item drops).
Summary: I started Act 2. From there, without progressing the Main story (talk to takamora), went to delemain and did his quest. The file jumped in size from the act 1 final mission to act 2 by about 100kb. Then, doing delemain's quest, that file size.
During "the Heist" = 2119kb
Act 2 apartment = 2271kb
Epistrophy finish (delemain quest) = 2997kb
Yes, it has been stated, playing the game increases the save file size. There is no refuting that.
However, I mentioned before, I think that "discovering" each new location adds a chunk to the save file. Simply noting the before and after doing a single quest, that takes you through each new area.
As you can see with the next saves, which were made in previously discovered locations, the file size does not increase much simply by completing a quest. Though there is a change in file size based on the location, I think.
1- Finished quest "heroes" (Jackie) - Manual save outside the bar = 3063kb
Then, I fast traveled and saved in the apartment:
2- Finished "Heroes" (Jackie) = 3051kb
So, while driving around for delemain, I "discovered" new locations for the first time. Doing so jumped the save file massively. Now, some might point to the quest completion doing that on its own, but completing the following quest sort of invalidates that. Having completed a quest inside of an area that is no longer "new", the save file changed by only about 60kb (depending on location). This quest is one of the more "main stream" side quests rather than a true side quest (gig, ncpd, etc) so perhaps that affects the file size more than some of the smaller side quests such as "kill enemy A", which I've noticed while playing through act 1 only affected the save file marginally. This lines up roughly with how much the file increased when completing the act 1 quest, which was a true "main" quest.
Attached is the map of watson showing that while I've done a number of side quests there (mostly ncpd), I've not cleared them out completely. This was all done in act 1, which brought my game to 2mb prior to "The heist".
ALSO, do note, that I pick up EVERYTHING. So considering I picked up every item not nailed down through the end of act 1 mission, the file only increased ~100kb. Whereas each "new" location I "discovered" added ~50kb-100kb (Based on 7 major areas with subsections in them increasing the file by ~700kb: Watson, Westbrook, Badlands, Santo Domingo, Pacifica, Heywood and City Center)
In conclusion, through normal play, it appears you are going to see jumps in file size because of Main story quests, "main" side quests and "discovering" new locations for the first time. Besides those, the file more or less stabilizes. Which lines up to FlorettaV's save file size (6ish mb?). However, that's with minimal crafting and no exploits iirc.
This information doesn't pertain to somehow being able to craft to your heart's content without bloating the game, but figured it would be useful to someone worried about their game file size.