a) being bigger means more technical complexity, more bureaucracy, and more risk, all of which mean each individual employee becomes less productive
b) there's tons of features that their products tend to have that a typical user doesn't know about or care about, which people aren't factoring into their estimate of how many employees the company actually 'needs'
c) there's tons of aspects of a software product that increase in importance the bigger you are that aren't your core thing, but still need to be taken care of: internationalization, security, privacy, accessibility, etc. Of course the typical HN take is probably "psshhhh, that stuff is important no matter how big you are" but realistically, a small startup is gonna spend less time total on those things than a big mature company, most of the time