This issue has been reported before but the discussion halted because it is deemed out-of-date or not reproducible. Importing small assets every time I save a scene - Questions & Answers - Unity Discussions
It is not out-of-date and is very much reproducible.
It’s an annoying little message that occurs 80% of the time(s) when you save a scene, but not always. It also seems to occur sporadically (1%) during editing. The vagueness of the message “importing small assets” makes the Unity3d user experience feel like it’s sick, infected with a virus or spy or hack, or playing a secret joke of some long lost programmer. If it’s not a bug or a virus or a third party spy, then I vote to have it either edited to explain which asset is being imported, or, removed entirely. It seems to suggest that Unity is downloading assets without explanation, without authorization, etc. (which is atypical of Unity everywhere else) and this makes the user feel less confident in the product.
I used to believe it had to do with importing from the Asset Store. I do not believe this anymore because my current project has not downloaded any assets. I also do not have any plugins. Perhaps it’s the lights, which seem to behave differently than other 3d objects, so perhaps they are the cause and need to download something.
Please up-vote this issue!