The maximum fine the EU can impose with the DMA is "10 % of its total worldwide turnover in the preceding financial year". I think this would hurt even Google. And I think a billion dollars is not nothing.
EU competition law is not framed around valuing Europeans more than others, it is framed around protecting competition and market functioning inside the EU.
No need to moralize it and pretend anyone is using this "value" European humans more than other humans.
Why not 20% (30%), given that's the share of world GDP (PPP) due to the EU?
Why revenue rather than profit?
The answer is: big round number that's roughly justifiable as a big stick for, and I want to emphasise this, a punishment to make sure they don't break the rules.
The reasoning is very simple just "carry a big stick", multinational companies need to fear the fines to want compliance.