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So either the numbers I remembered were a bit outdated, or were using a higher cutoff than 250 (probably 500, since I remember it being not that much less than 50% working at "large" corporations).
I'm looking at "Statistics about Business Size", and considering as "large businesses" those firms with 500+ employees, as that's the largest top-level bracket they track. Within the 500+ bracket, firms with 5000+ employees are (for whatever it's worth) the largest component by far.