Let's simplify for clarity that tougher is strongly connected with the size. To me the boundary is 4-1/8". The 4-1/4" or more are too long for pocket carry, even if the knife is thin. I am puzzled how is possible 1/8" to make such a difference, for knives over 4"?
Anyway, this is based on my experience. I have carried the Mercator and the big Squirrel only once and twice. The Sodbuster and Sperapint ( both Friedr. Herder, 4-1/8") are noticeably better for EDC. In my experience the tickness of the handle is not so important and critical than the 1/8" mentioned above. And, of course, the small Squirrel is good enough and tough enough (only the blade shape distinguish Squirrels from Douk Douk) for EDC. I do not have any knives with pocket clips, and the Mercator and Opinel #7 are the only lockinng folders I have. Ohh, and I never entertain the self-defence idea.
Anyway, this is based on my experience. I have carried the Mercator and the big Squirrel only once and twice. The Sodbuster and Sperapint ( both Friedr. Herder, 4-1/8") are noticeably better for EDC. In my experience the tickness of the handle is not so important and critical than the 1/8" mentioned above. And, of course, the small Squirrel is good enough and tough enough (only the blade shape distinguish Squirrels from Douk Douk) for EDC. I do not have any knives with pocket clips, and the Mercator and Opinel #7 are the only lockinng folders I have. Ohh, and I never entertain the self-defence idea.