I'm not sure what it is with pads, but somewhere I read that a menstrual cup creates a mild negative pressure that helps to pull the flow out. That also explains why I have less cramping using a cup than with any other method: the uterus doesn't have to work as hard to get the flow out.
When I changed from using disposable pads most of the time to using tampons all day long, when I was 14, my periods increased from 5 to 6 days, and I started to see more and larger clots. At the time I thought it was just that I was making thicker linings as I matured. Now I wonder if it might have been because the tampons blocked the flow, encouraging clots to form, which further blocked it.
It's strange: I loved tampons once I became adept at using them; I thought it was so cool to be able to "take control" of the flow and not have to feel that yucky dripping sensation. I was totally satisfied with my feminine hygiene method, and when I first heard about cloth pads, I thought, "That may be better for the environment, but I don't like pads." It wasn't until (a) Kotex stopped making their stick-applicator tampons, and I couldn't find any others I liked as well, and (b) my environmental sensitivity reached the point that watching the bathroom wastebasket fill with menstrual debris made me feel queasy, that I began wondering about other options and did a Websearch for "reusable tampons" and found the Keeper. Once I had my Keeper, I lost interest in tampons so completely that I STILL HAVE THE SAME BOX FROM 1997, HALF FULL!

It blew my mind how many of the discomforts I'd assumed were inherent to menstruation actually were linked to disposable products! I still thought I didn't like pads, until I won a free cloth pad (by submitting my cramp-relief tip to a Website) and thought I might as well try it...and found that that yucky dripping sensation isn't nearly so noticeable with a pad that isn't engineered to suck every bit of moisture out of your genitals!

Now, when I think back to my menstruation experiences as a teenager, I feel sorry for myself!