I come at this question from a backpacking and climbing perspective. Bulk and size matter. You want the most compact tool that will get the job done since you will be lugging it around.

I have found that anything under four inches in blade length will do any necessary task just fine. I have never thought - "If only I had a bigger knife." There have been times I wished for a saw, but even those were rare.

I suspect there is a Freudian explanation for the fascination with big knives.