Yes, they do. They're not even just the largest employer bracket; they literally employ a majority of the employed in America.
http://www.bls.gov/bdm/entrepreneurship/entrepreneurship.htm
Firms with 250+ employees have been slightly >50% since 1996.
http://www.bls.gov/news.release/cewfs.nr0.htm
37.4% of employees are at firms of 1000+, 6.8% are 500-999, 7.2% are 250-499.
So either the numbers I remembered were a bit outdated, or were using a higher cutoff than 250 (probably 500, since I remember it being not that much less than 50% working at "large" corporations).