I don't think she thought them out carefully. I just think she wanted to make them big and heavy. I think the Bludgers might have most of their weight distributed along their outer shell, and they might be internally hollow. I think the bats are the thing that concerns me. How could someone use a bat heavy enough to hit a Bludger? I suppose some of the answer has to be 'magic.' I think that's why people don't die more often, actually, because the Healers are able to mend bones and more serious injuries that would kill Muggles.
So, do we just pass off the dimensions of the Bludger as something Rowling didn't think very carefully on? Or is there some other explanation for why such large Bludgers don't kill people (supposedly)? Could they be hollow?