How does Avast decide WHICH downloads to scan?

Dear Avast.

This is NOT a complaint, just a question!

Went on a legit. free download spree.  One of the downloads was pretty big - 5 .RARs of stuff - and  the other 2 were relatively small, just free VSTs.  The first one was a legit. free DAW.  

Now the downloads have just finished.  The first one, the DAW, is being inspected by Avast through a microscope, I mean it's on 2 hours plus and still going strong.  (That's the big d/l!) What's puzzling me is it's not scanning the second two at all.  I don't mind it taking forever scanning the big one, as long as it stops at SOME point, but what made it so sure the two smaller ones were clean without even scanning them at all?  What's its criteria?

I don't THINK it scanned them so fast I didn't notice because, as it's so busy on the big one, wouldn't it have queued the others up behind it?  And more worryingly, if it's because the other 2 are a lot smaller, what if I downloaded something that was small but DID have a virus in it, or a gate to let a virus through once installed? How would it know if it decided it was beneath its notice and didn't scan it? 

Yours puzzledly

Chris.