I have several dive knives, mostly blackie collins wenoka knives and benchmade. Most of my dive knives are larger in part because they were acquired in the era when a big knife was the norm, rather than the exception. I continue to do it today because if find the larger knives easier to handle with drysuit gloves and the simpler the release mechanism the better.
Diving is ultimately an exercise in self-expression, one of the few less regulated activities we are allowed to undertake. If I want a big knife, why should anyone care. There is an undercurrent of political correctness in this thread that I find amusing. I dive with the equipment that I choose to dive with and unless there is a local restriction it is nobody's gd business what I chose to use. if you don't like it, pony up the bucks to buy me the knife you prefer.
I was on a diveboat for five days in hawaii. I had my big knife with me because, as I mentioned above, I usually dive with insulated dry gloves in 36F water. The instructor on board took me aside and explained that most hawaiian divers dont carry knives because there is no kelp unlike the coastal areas of the west and Alaska and the environment doesn't call for one. I appreciated her observation and explained that after 30 years of diving with one, its just part of my rig. We left it at that, no judgement, etc.
As to the quaker thing mentioned in a couple of the posts. Diving in remote areas with tidal swings and currents, sometimes adverse meteorological conditions, and what not, I find it is better to be prepared rather than not, which is why I also carry a safety sausage.