Basically, the story goes that when an individual rises into a significant leadership position at a large enough company that the economic calculations become different. There's still an element of domain expertise, but, for the most part, leadership is leadership wherever you go. This implies that a leader could (potentially) move across sectors and still be effective which results in a wider pool of companies that are interested in competing for this person when contrasted to the ICs. Since some sectors are very profitable they end up "bidding up" quality leadership. The combination of this effect along with the fact there are objectively fewer CEOs than ICs results in a mismatch in salaries.
I think there's an element of truth to this, but probably not to the extent that it justifies the widening pay gaps everywhere?