It changes for me all the time.
Right now, I'm carrying a Leatherman Juice ES2 on my keys, which is a keychain sized multi-tool with a wire strippers as the main tool. (I'm an electrical engineer)
The "proper" pocket knife in my pocket depends on what I'm expecting for the day. If I'm expecting to sit at my desk all day, it'll probably be something small and simple, like a Peanut, Pen, small toothpick, or even a Vic Classic. If it is the weekend and I'm going to be doing handyman-type stuff, it'll be a Vic Cybertool 34 or Vic SwissTool. If I'm going to the woods, maybe a Sodbuster Jr., Vic Soldier, Slimline Trapper, or even maybe a full size Buck hunting knife. (which doubles as a small machete) If I'm not doing anything special, something of medium size.
Right now, I'm on vacation, helping my wife out with the newborn. Around the house, I sometimes don't carry anything, as there are knives scattered here and there throughout the house.
As a knife collector, there's no reason to feel obligated to stick with a certain combo. We have dozens of knives because we like the variety. At least I do. Everytime I try to pigeonhole myself into some kind of rotation or pattern, I get anxious and I'm dying to switch. So I've given that up. My taste in knives is kind of blue collar; my most expensive knife is the Vic Cybertool 34, at something like $85. I've been collecting them since I was 13 or so (I'm 35 now) so I have lots from which to choose.
FWIW - I just got an Opinel the other week. I lost my first one, which was a #7. I consider that medium size. The one I just got is a small/medium #6. I consider #8 to be medium/large. I love how light this knife is for the size, and I love that it feels so warm in the hand, even if my leg hasn't been warming it up in my pocket. I love the rotating locking collar, and the fact that it is basically the same after 120 years on the market. I also love that the blade isn't overly thick. It is thin enough to still be useful for food prep. I can't say that about many of my pocket knives. I've got to give the French props for respecting their traditions.