It's way more complicated than that. Antitrust law doesn't care about "the market", it cares about "the relevant market", where "relevant" is a legal term of art that may or may not have anything to do actual markets.

There were 6 factors considered in establishing the relevant market in the Microsoft case (paraphrasing Court's Findings of Fact, §2)[0].

A) Servers were not cost-effective replacements for consumer hardware and were excluded from the relevant market. It's not clear whether the discrimination also goes the opposite way (e.g. instead of "smartphones" we compare "flagship smartphones"). If so, that kind of market definition would really make iOS look gigantic.

B) PowerPC and other non-Intel-compatibles were excluded from the relevant market since consumers could not install the competing OS on their hardware (that's right, "the relevant market" was constructed so as to exclude Apple, MS's only serious competitor). It is an interesting question whether the fact that you can't run Android on your iPhone for non-ISA reasons would be viewed similarly to an ISA difference in the hardware, but the way I read the ruling, it seems likely to be viewed similarly, or perhaps much worse. The reasoning all stems from whether consumers have choices to install their own OS, nothing to do with ISAs per se.

C) Various "borderline formfactors" were excluded from the relevant market. In the 90s this was e.g. WebTV, thin clients and game consoles. Today this might be things like tablets, VR platforms, chromebooks, etc. These are (today) all Android form factors, and excluding them to focus on "traditional" formfactors makes iOS look a bit larger than it does in Netcraft surveys.

D) The court observed that market conditions create a feedback loop, e.g. more people write apps for the dominant platform, that make the dominant platform more attractive to write apps for, round and round it goes, etc. This is obviously true for the smartphone market.

E) The court was concerned that nobody could realistically enter the market and "in less than a few years, present a significant percentage of consumers with a viable alternative to incumbents." This seems probable for the smartphone market, based on Mozilla and Canonical's misadventures, RIM and Nokia's inability to make a comeback, and even Windows Phone.

F) There was also a discussion about why "portals" (like today's cloud computing and/or webapps, basically) are not an effective substitute for doing computing locally and should not be in the relevant market, as well as a discussion of why Java is not a substitute for Windows (obvious to us, but not to lawyers)

All of this to say, it is really not so obvious from a legal POV why Apple are not a monopoly or are in any way different fro the Microsoft case. The real reason is probably nobody wants to sue them; I have seen estimates of the Microsoft case that suggests MS spent more than $500M defending it, and the taxpayers (with much cheaper lawyers) spent more or less $100M to prosecute it and didn't get much for their money.

Then there is the question of who gains by spending that money: Apple is well-loved by their customers so it doesn't have the election appeal for politicians that the Microsoft case did. Google would prefer to implement Apple-like policies in their store and don't want them found to be illegal. That leaves bit players like Canonical or Mozilla, and for them the legal costs are likely prohibitive.

[0] https://www.justice.gov/atr/us-v-microsoft-courts-findings-f...