To dive into that.
Is fairly complicated. Again, while I know a little, most know more than me. But I'll try.
Consider the sparsebundle noted at https://discussions.apple.com/thread/1631502?start=0&tstart=0
It's a bit of a mystery to some but I "get it" why we don't want to have this bouncing around in size. Maybe Apple has an article on how to tell it to compress that by releasing empty space in the bundle? Sorry but this sounds a wee bit like a high wire act. That is, I don't want you to lose your data in that backup.
I find many don't backup beyond this so any tinkering is off limits until we have our second and third backup copies of what we can't lose.
Hope this helps,