There are not only monopoly rules. It seems plausible to me that there could be a rule that mobile phone/computer ecosystems above a certain threshold must grant access to the platform (under reasonable conditions).
That would be the EU going beyond their reach, invading into private business practices, something more akin to the Soviet Union than the EU. Apart from that, "size" means nothing and is completely arbitrary; the EU has only ever really chased monopolies and companies that flout EU regulations and taxation. Let's stop injecting our own ideologies into what we'd like some state or other to do; we should never want any kind of government to regulate that heavily.
