Ok I get it now. I started working on this map in december I think and have been updating it regularly. I remember there was this time when it went from 40-50K to 99999 all of a sudden, months ago, and it was quickly fixed. Then yeah a few weeks ago it was around 76K and that's when it jumped back to 100K. I figured this time was the same bug and would be fixed the same way, I didn't think the real memory would be even this high I thought the bug was actually displaying the wrong budget not the map being over budget. At first I was just deleting stuff to make room and replacing it with new additions. At one point I discovered this bug : https://new.reddit.com/r/FortniteCreative/comments/b66jgh/for_those_who_experience_the_memory_increase_bug/
And started using this method to be able to keep working thinking I was still in the limit because the TRUE memory would be around 76K as it was before the bug. So from this point I kept adding stuff until you explained the issue.
If I had to guess, I'd say after being over budget I added maybe 10K more memory so maybe the initial change brought the memory from 76K to like 120-130K.
As for the island as a training island it features a lot of unbreakable gallery structures (walls, floors, ramps), builds. A lot of sentries, item spawners and quite a bit of bouncers as well. I checked a few of those to see what would've caused the memory to jump and didn't see any big difference in size. I just noticed recently tho, the unbreakable gallery structures, sometimes would cost 3, sometimes 4 memory. Don't know if this is supposed to happen, not sure if this was already like this before but if not this could also partially explain why the memory increased.
But also while deleting stuff, I saw something strange happening : Deleting bumperds would make the memory go down crazy fast : https://streamable.com/m5ayu
Maybe there are some other devices glitched ?
Anyway, I was indeed able to go (barely) back under budget thaks to your explanations.
That being said I'm now very close to the limit while having deleted a lot of stuff on my island. I see that you're gonna try to avoid things like that happening in the future, but can we expect what was increased in size reduced back ? Or yeah being able to know if there are changes of this type because even after having deleted so much I'm still higher than I was before and I haven't been able to figure out what made the memory go up.
I think I made in-game feedbacks that might help as well.