Cabin size is sufficient. The problem with size is patient girth, as Devil49 pointed out. That's the only thing that really matters.
Using one big helicopter instead of multiple smaller ones isn't viable economically. You can't plan for a situation that happens only a very few times per year, you have to plan for the everyday case.
Carping about the economic model in the US does no good. I would also like to see the entire medical industry model change, but it's not going to happen. Capitalism will continue to rule, so we need to find the best way to deal with that and its effects. The FAA is required by law to consider economics in its regulations, and can't impose any that would cause severe economic penalties, regardless of whether they would save lives. It's wrong, but it's the reality.
I think the number of night accidents reflect both the numbers, and the relative risk. I almost always had about as many night as day flights, and I suspect the numbers are at least on the same order of magnitude with most programs. But night EMS has its dangers. It's dark. That makes it hard to see. Duh. But it's true. Googles help a lot, but they have their limitations. Everything is one color, and the field of vision is very narrow. It's really easy to miss seeing something that's just a few degrees to one side. The biggest danger, IMO, is the physiological difference. If you're asleep, and wake up from deep sleep for a flight, sleep inertia is difficult to overcome, and can take a long time to pass. Pilots who live locally, and stay on the same diurnal schedule regardless of the work schedule are more likely to be affected, and are also more numerous. Companies want to save money by having crews live locally so they don't have to provide quarters, and pilots don't like being away from home for a week at a time. IMO, this causes accidents, and lives. But it won't change. Money talks.
Night HEMS accidents could be completely eliminated, of course. Just eliminate all night HEMS flying. Lots of people across the pond advocate that, and some don't allow night flights there. But one has to take a broader view, I believe. Of course people die in night HEMS crashes. But the real question is whether those deaths outnumber the lives saved overall. That's a difficult number to quantify. But one still needs to consider the tens of thousands of night HEMS flights annually in the US, and the relatively small number of accidents. I'm certain that night HEMS flights will continue, and continue growing. How to properly regulate them, and minimize the risks, is the question. We will never eliminate risk, we can only minimize it.