I use qcow2 file format and create snapshots using a Virt-manager GUI. 
I assume I have 100GB disk space and 70% is used by VM at this moment. The qcow2-file size is about 70GB. I create a snapshot. It is stored inside the qcow2-file, we know. After some days the qcow2-file size is rised up to 120GB. OK, we understand, why. I delete this snapshot, but the qcow2-file size does not change! 

"qemu-img info" does not show me any snapshots for this qcow2-file! How to see, what's inside this file and how to clean it up? 

I've used "qemu-img convert" but I have a feeling inside that it's not the rigth way to decrease the file size.