Well the system volume information folder can be absolutely huge unless you restrict its size in the system restore. I have seen it balloon to over 1.5GB. The progress isn't a precision tool and I think it updates after folders, so that may account for the jump.
There is nothing in the program to make it skip the system volume information folder/s unless you specifically exclude it from scans, unwise.
Right click on the folder and see how big it is ?
The boot-time scan I don't think scans all files. I don't see this huge jumps on my system, for one I don't do a thorough scan with archives, life it too short.
Archive (zip, rar, etc.) files are by their nature are inert, you need to extract the files and then you have to run them to be a threat. Long before that happens avast's Standard Shield should have scanned them and before an executable is run that is scanned. Thorough is also by its design very thorough and perhaps a little overkill for routine use, were a Standard scan without archives should be adequate.
I have only ever done a through scan with archives once shortly after installation just to ensure a clean start state, but with XP for example avast will do a boot-time scan after installation if you select it, this I believe will be quicker and reasonably effective. Like everything in life things are a compromise.