tl;dr:
critiques of small business aim to reject the following dilemma as false,
> The alternative [to monopolists and global megacorporations]? Small business.
rather than to promote big business.
In the USA, the Bureau of Labor Statistics calls any business with 500 employees or fewer a ‘small business’, and it's usually something like that which gets translated into policy. Not all that small, imo.
But absolutely people valorize small businesses in a way that elides the fact that what makes businesses exploitative is (contra this former Amazon VP) inherent in wage relations, and smaller employers are still operating under those relations. Because small businesses are romanticized and valorized as they are in their culture, the language of ‘promoting small businesses’ is often used to give cover for massive corporate giveaways on the part of the government. These are [common](https://inthesetimes.com/article/22412/outrageous-corporate-...) [lines](https://www.currentaffairs.org/2018/07/can-business-of-any-s...) of [socialist (this piece has some of the exact language of the grandparent poster)](https://jacobinmag.com/2018/01/small-businesses-workers-wage...) [critique (see Morales on free trade here)](https://inthesetimes.com/article/22412/outrageous-corporate-...).
Generally, calls to treat ‘small business’ with greater scrutiny come from an anti-capitalist (or at least ‘capitalism-critical’, if you will) perspective, and are not motivated by some belief that monopolies or anti-competitive cabals are actually preferable to larger numbers of smaller, weaker, relatively competitive corporations. But there's a sense that the critique should go deeper than ‘big business bad, small business good’— which imo is pretty commonly distilled in a lot of mainstream political rhetoric— and raise questions of worker ownership, decommodification (e.g., of healthcare), the role of public enterprises, etc.
Advising skepticism toward the valor of small businesses also doesn't negate the fact that promoting small business _can_ be [beneficial or strategic for society as a whole](https://inthesetimes.com/working/entry/22385/coronavirus-wor...) in particular circumstances! But I sympathize with the grandparent poster's general sentiment; I feel like on the whole it's called for.