ITIL states that the ECAB is a subset of the CAB. Why is that the case? Why is it not a superset including more members to make sure communication with external parties is more efficient?
Based on this article, I would say it's a subset because when dealing with emergencies, only the most important steps are taken, in order to solve the problem ASAP.
The idea is to solve the emergency as fast as possible.
E.g. during ECAB not every possible factor is taken into consideration and we don't analyze all possibilities.
Adding more people, as you suggested, will add overhead management and coordination; anything that delays the solution is unacceptable.
To quote the abovementioned article:
The ECAB is a subset of the CAB that is responsible for changes that are considered emergency changes. While the CAB takes every factor under consideration and analyzes all possibilities, the ECAB tends to work under a much stricter timeline and focuses on risk analysis and minimization. Time is often the most valuable resource in the case of an emergency and one that cannot be squandered.