For me it has a lot to do with what the knife is made of more than the size or thickness.
I would feel safe with the knife on the right, the mighty rat tail at 3 3/4" long and 3/16" thick.
Yet I would not feel safe with the knife on the left. It's actually larger with more handle space, but I don't trust 154cm even with the absolute best heat treat possible. I've seen to many chipped blades in that steel, and too little "flex before it breaks" warning for the user. I've used the rat tail for fire building and it's a GIANT pain in the butt trying to break down large hunks of wood with it, but it manages.
I found this knife to be the best knife for wood shaving and carving out of anything I've used so far, and I'd actually feel safer with it than the swamp rat harry carry shown above in 154cm because I know what happens to it when you push it to far, it bends and takes a set. I
know that it would take less striking force to become damaged than the harry carry, but I know exactly how much force I'm allowed.
Give me a knife in s30v and I won't feel safe with it no matter how thick you make it because I simply don't trust s30v. Give me a knife in INFI (or A2 heat treated for toughness) and I'll feel safe with it down to 1/8".