A wound can be larger underneath than the opening you can see. Wound care instructions assume this. When you pack a wound with moistened gauze, you carefully fill in the wound and any spaces under the skin. That step exists because spaces under the skin are a known part of some wounds.
Those spaces matter because of the order healing follows. An open wound will heal from the inside to the outside. A dressing helps absorb drainage and keeps the skin from closing before the wound underneath fills in. A space under the skin needs to fill in with new tissue before the surface closes over it.
That filling-in is part of normal healing. The wound starts to fill in with new tissue, called granulation tissue. New skin begins to form over this tissue. As the wound heals, the edges pull inward and the wound gets smaller. A surface that closes while a space remains underneath is out of that order.
Why a space forms beside a wound, how to measure one, and what size is worrying aren’t things to work out at home. A space under the skin is something your provider finds and then gives you instructions for. If you have found a space under the skin that nobody has mentioned to you, that is a reason to have the wound looked at.
Do not pack anything into the space yourself unless your provider has taught you how, and do not push a swab or anything else into it to see how far it goes. Dressing instructions like the ones above are for people whose provider has already shown them the technique and told them how often to do it.