A market the size of the US or the EU is large enough to sustain a domestic industry. Competing suppliers can then recover their fixed costs from the markets where they're protected against monopolistic practices, which sustains their existence, and then sell into the other markets with the backing of a stable competitive domestic supply chain that can't be degraded by anti-competitive practices.

Example: Right now if you want to create a competing phone without Google, you lose most of the apps in the Play Store because they're encouraged to have dependencies on Google services. If that was prohibited in a large market then app developers couldn't assume those dependencies are available and the APIs would either have to become an open source part of stock Android or developers would use platform-independent third party libraries. Then the apps made for that market could run on a third party device -- but developers aren't going to make separate apps for different markets, they're just going to avoid the proprietary APIs that aren't available in a major market, and then those apps don't have those proprietary dependencies in the other markets either and your domestic competitor can compete there too.

Also, the whole world outside the US and China wouldn't even have the excuse of protecting domestic megacorps as a reason not to have strong antitrust laws themselves.