It could be that the file was compressed before, and you saved it uncompressed.
In my experience, the file size closely matches what I’m doing.
It is possible that you did some experiments where you temporarily created some meshes, materials etc. that take up a lot of memory. Later on, you deleted the object referencing this data, but AFAIK Blender cleans up the referenced data only when opening a file. So you could try to reopen the file and save it; possibly it gets smaller. You can use the data browser (SHIFT-F4) to search for data with an owner count of 0. This data will be removed the next time the .blend file is opened.
You can also look for data with an owner count of 1 and which is marked with an “F” (“fake user”). This data is not referenced anymore and only kept because you told Blender to keep it even in this case. Select it and press “F” to remove the fake user. See the manual on Blender’s and data system.
But generally speaking: Without the .blend files, one can only speculate, unfortunately. Maybe you could upload them somewhere, so someone can take a look.