Why larger would cannot be heeled immediately????????

The correct spellings are 'wound' and 'healed'.

When major injury occurs, tissues at that place swell, new blood vessels are growing into the wound and collagen as well as extracellular fibres get deposited. Together, the newly forming cells, blood vessels, and loose extracellular matrix are called granulation tissue. Granulation tissue fills the base of an open wound during healing.

In  small wounds, restoration of tissue continuity occurs directly, without granulation. However, in case of larger wounds, wound  repair is accomplished by closure of the wound with granulation tissue that will slow down the healing process. 

However, healing of wounds depends on many factors like location of wound, severity of the wound, extent of injury to the tissues, the age of injured person, his nutritional status, and general state of health of the patient, and available body reserves and resources for tissue regeneration.