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Wayne and Orion are walking along a country footpath. A friend is metal detecting in the field beside them.
Wayne: Morning!
Friend: Morning.
Wayne: Why is old stuff buried? And why is the deeper stuff usually older?
Wayne: Is the world getting bigger?
Orion: Several processes can bury things. Rivers leave silt after floods. Wind deposits dust and sand. Soil builds up, and people build over the remains of earlier buildings.
In an undisturbed sequence, the lower layers were generally there first. But digging, ploughing, roots and burrowing animals can mix things up. Depth alone cannot give you a reliable date.
Meanwhile, erosion removes material from other places. Much of what accumulates in one location has simply been moved from somewhere else.
On a much larger scale, new ocean crust forms at mid-ocean ridges, while old crust is recycled into the mantle at subduction zones.
So a buried coin does not mean the planet is swelling. It means the ground around it has a history.
Wayne: Oh. Just wondered.
They carry on along the path.