Growing scene size despite removing objects

I've got a scene that just over a gig now and to try and reduce the size a bit i've started making some prefabs out of objects and just instantiating when needed. However the .unity scene itself hasn't reduced in size, if anything it's gone up a little!
It's like things are being left behind and not cleaned up, the project has been ported from 5 to 2017 and newer scene don't seem to have the same problem...

If anyone has any ideas how to reduce the size or force it to re-save or something that would be great? !

 

Found out what it is. We're using the Adventure Creator plugin and have a bunch of Trigger objects that seemed to have gotten bugged along the way and duplicated the actions within them 100's-1000s of times!