I am wondering, why do big, deep wounds leave scars and small ones do not?
Most wounds leave some kind of scar. It's just that we sometimes can't really tell because it's so small.
Here's how wound healing generally works:
- Blood clots and forms a scab (this may not happen, for example in burns and puncture wounds that didn't draw blood
- White blood cells (macrophages) "clean" the wound - the wound may appear to be oozing some fluid. Blood flow increases, the macrophages produce growth factors.
- Blood flow increases more, red blood cells deliver more oxygen. Cells at the edge of the wound secrete collagen. The wound fills with tissue called granulation tissue.
- New skin grows over that tissue.
These stages are called hemostasis, inflammation, proliferation, and remodeling. This process can take weeks to years depending on size and location of the wound.
Now, what's a scar? Scars look different from "normal" skin because of collagen. Collagen is a protein on the outside of the skin cells holding them together and making skin strong (it's also a major part of tendons). In normal skin the collagen sort of forms a "criss-cross" pattern, while in scar tissue it's more parallel. That probably comes from the wound healing process.
In small wounds, you won't notice that the skin looks different because of that. In larger wounds, it's very apparent at first, though scars also start looking less noticeable with age.
Factors Affecting Wound Healing (introductory section)