Modern big corporations are not bad because they are “big” (that’s a symptom and not a cause). They are bad because they are “corporations” (i.e. incorporated into a State) and State-protected and State-privileged “corporations” are an integral part of the historical and present Statist order (every society in which governments exist).
Of course, modern corporations are likely also unnaturally big, by virtue of their unnatural monopolistic (or, oligopolistic) privileges. Only after we enter Statelessness (anarchy) would we truly empirically learn just how big a natural enterprise can become.
Having said that, the flip side is also important to note : in today’s Statist world, merely not being “big” and merely not being a “corporation” (narrowly defined) does not absolve an enterprise fully — small LLCs and even sole proprietorships that are registered and contracted with the State are also morally complicit, even if to a MUCH lesser degree. Only a fully agorist enterprise or economic transaction, that operates in the so-called “grey/black markets” (more properly seen as the only “free” portion of today’s market), can be said to be fully free from sharing in any guilt associated with the State’s bloodied loot.