I have to admit, size is one thing that has always bothered my "realism radar". A halfling using a dagger just can't hurt a huge creature, no matter how skilled he is. It's not a question of knowing the right techniques or whatnot, it's pretty simple physics. A five inch knife is not going to hurt a dinosaur.
What makes you think that? You say it's physics, but really, we're talking about biology.
Take an elephant. Elephant's have incredibly think skin, about 1" think. Plunge a 5" dagger in, and you have 4" of penetration beyond the skin. Don't forget that a large animal means large organs.
To me, saying something like "small creatures can't defeat a huge creatures with daggers" sounds more unrealistic. As though there are no circumstances that would allow it. And even if there are circumstances, they're so remote that they can't be repeated by skilled and powerful small creatures.
But, by and large, this is just one of those things you swallow and move on. If I wanted realism, I wouldn't play D&D.
Understandable, but it seems to me that this thread is about dispelling claims of "unrealism" by people who make intuitive, but erroneous, conclusions about the world. Reality is often counter-intuitive.
Oscar Wiled once said "Man can believe the impossible, but man can never believe the improbable." I think a lot of DMs fall into this trap, confusing improbable with impossible. In addition to the oft discussed "you can't do that" there's the "oooookay, a guess you can roll it" whenever a player wants to use a skill that they don't have a high bonus in.
Of course, I see the reverse in players. Because they have a low bonus, they don't ever want to use it. To the point were it never occurs to them to use the skill, even when it's the most appropriate thing to do.