From what I can get this just gives you more attack damage the more health you have but, I'm not sure
The passive of the item does not increase your attack damage. It increases your autoattack damage based on the amount of health you have. When you autoattack, you'll do a small percentage of your maximum health to the guy you hit, and a larger percentage of your maximum health to everyone behind them. What this means is that it will make your autoattacks stronger if you are tanky, but your AD-scaling abilities will not be more powerful when you get tankier.
- I have no Idea whatsoever about what this does
As you move around, you start moving faster and faster (up to a cap). If you autoattack someone, you lose the extra speed and do bonus damage and possibly slow based on how much extra speed you had. If someone hits you or crowd controls you, or if you stand still, you lose the extra speed very quickly.
What does increasing in size actually do?
It makes your model and hitbox larger, which makes you easier to hit, which also makes it easier for you to tank skillshots. For a champion that is meant to soak up damage, being able to absorb more incoming fire can be a very good thing.