I know, I had the same problem!
I think blood looks heavier on a tampon than on a pad. So, in reality I think my flow was pretty light on my regular periods even though my tampon would look saturated. So when you equate that to a pad, it looks very, very light.
During my miscarriage I was actually surprised at how light the flow was.
I'd say light flow would be not needing to change it for hours...like over 6 hours (even though you would probably change it to feel fresh), and heavy flow would be you WANT to change it every two hours or so.