My gut would be sheer size of the person? Though, there are obviously a multitude of things that could be different.

Average body mass would be my guess.

So we could tease this out by taking 90-110kg men and comparing to 90-110kg women?

Seems reasonable. Or do the whole analysis with the alcohol consumption adjusted per kg and see if the gender difference goes away.

Women cannot process alcohol as quickly as men.

That's a very good question that we likely can't answer right now. "Woman" and "man" encompass a very wide range of bodies, with different averages, but whose distribution overlaps immensely on nearly every metric.

These gender categories are standins for some subset of differences that are actually causal, but we don't have the measurements and experiments settings to pull out what is actually causal from this.

We have a clear understanding of why the difference matters here, see my other comment in this thread.

But in order to "know" that we need to do the proper causal analysis that shows that weight and alcohol dehydrogenase expression levels completely explain the differences.

Two main things. One is that women are smaller and body mass matters a lot to alcohol metabolism. The second thing is many women lack an enzyme that helps metabolize alcohol, which makes them even slower to metabolize it.