The Earth's Tectonic Plates are in constant movement, since they are not solidly attached to each other, and since they are floating on the molten rock underneath. This means that the plates are always either pushing against or pulling away from each other. The ocean floor is composed of inner-crust material that is released by this plate movement; some oceans are growing in size because their plates are separating, meaning that the total ocean floor is increasing. This is called a Divergent Boundary, where the plates move apart and allow molten rock to rise up and be cooled by the water; this also means that the ocean floor is collectively younger than the above-ocean landmass. The entire process is called Sea-Floor Spreading.
To simplify, Divergent Boundary movement increases the size of the ocean floor, allowing the ocean itself to become larger. The Atlantic Ocean is a good example of this, as the South American Plate moves away from the Mid-Atlantic Ridge.