I think size may also be construed as "infrastructure." For example, Facebook now has a couple of billion users. It has reached the point where we often have to get Facebook accounts/pages, if we want to reach certain users. In short, Facebook has become necessary to "survive," in a sense.
That sort of puts it into the realm of a utility; like power or water.
The idea of a utility, is that it is deliberately allowed to become a monopoly, sometimes, with state enforcement. The flip side, is that it is now required to provide a lot of services.
For example, if some old folks can't pay their electric, in winter, the utility might be required to supply them with electricity anyway, and eat the cost, or claim it as a tax deduction.
That's the downside. The upside is...MONOPOLY, BABY! WOO-HOO! PAAAARTAAAY!
So there's a big carrot, as well as stick. People who own utilities tend to get pretty damn rich.
This all kinda breaks down, if the utility is already a monopoly, so the state assigning them monopoly status means nothing. No carrot; only stick. It also breaks down, if the utility manages to corrupt the regulators, thus eliminating the stick.
Facebook is already a monopoly. It doesn't need the state to give it anything; certainly not with a stick, attached. Thus, the "Standard Oil" remedy.
Apple isn't quite like Facebook, but it's getting there. The problem is that a lot of what gives its products value, is that iron-fisted control Apple has over their configuration. If that control is diluted, then it would also reduce the value of Apple products.