Well it is my experience that many US hosters are really small reseller accounts. There are a few key players but the reality is that the number of top tier hosters per country is fairly static in most mature markets and the US is a mature market. Thus (based on identified US hosters) you have over 100K US hosters with 1-9 domains hosted, around 60K with 10-100 hosted, around 12K with 100-500 hosted, about 1570 with 500-999 hosted, about 1390 with 1000-4999 hosted, about 170 with 5K-10K hosted, about 217 with 10K-50K hosted, about 34 with 50K-100K hosted, about 45 with 100K-1M hosted and 6 with over 1M hosted domains.
The break point between resellers on shared hosting would probably be in the 500 region. Then it moves into the dedicated server territory. The dedicated datacentre operators would be in the top end of the market probably around 10K+ domains hosted.
The most terrifying statistic is that out of 937839 hosters in com/net/org/biz/info/ie globally, 514005 are one hit wonders (only one domain associated with the DNS). These are typically shared hosting type accounts. Only 34367 hosters globally (excluding ccTLDs) have more than 100 domains hosted. The real hosting business is a lot smaller than people think.
Regards...jmcc