Well, I have an HDS U60 here in my hand, and it is only 3.25" long, but it's about an inch in diameter.
The newest HDS models are longer.
Since 3.25" is not very long length, as most lights go, I suspect that it was the diameter of the light which you might have found to be bulky-feeling.
All CR123 lights are going to be at least 3/4" diameter, with most being larger diameter than that.
So, I think you need to decide if thickness or length is more sensitive to your carrying.
If you can stand more length, but less thickness, you might consider a AA or AAA battery format instead.
If you can't stand more length, then you are already at nearly the shortest battery length available with decent capacity.
Regarding my opinion on the light that you could "depend on for your life", I think it comes down to HDS or Surefire, if it indeed needs to survive rugged encounters.
However, if "life depends on it" only means that it needs to turn on when you want it to, and doesn't need to survive rough treatment, then it opens things up alot more.
If you want an extremely short light, with just a little narrower diameter than a CR123 light, and it doesn't need to be able to survive a military battle, you might try a 4sevens Quark Mini CR2.
This light is only 2" long and 3/4" diameter.
This is a twisty switch light, not a clicky. But it's very small.
Quark Mini lights do sometimes have loose threads on the head, but they have great customer service, so if you get a loose one you can return it for a replacement.
I don't think you'll find anything smaller and brighter than this, unless you get a custom light.
It has 3 brightness levels with 3 lumens low, 40 lumens medium, and 180 lumens high. Very bright on high.
The low level mode will give you 1.2 days of light at 3 lumens on one battery.
I realize that this is a less common battery, but it helps with the size issue you are sensitive to.
If you could stand just a tad longer at 2.3", and very slightly thicker at .8", then the Quark Mini CR123 is a candidate, and has been mentioned by others on this thread.
These Mini lights are about the smallest you are going to find in production lights.
They might not be as rugged as the HDS or SureFire, but if size is the most important factor, then they are about the smallest. And they are reasonably durable if you don't abuse them.
The biggest complaint heard on these Quark Mini lights is loose threads on the head. It's not loose in all of them, so you can get a tight one.
Also, they are not very expensive, and the 4sevens customer service is good.
I'd still recommend the HDS for ruggedness, but for size, the Q-Mini are about the smallest.