As the bottom begins to interfere with the circular motion of the waves, it uses up some of their energy and begins to slow them down. When the leading waves slow down, the following ones push in behind, and the result is that the waves are squeezed together and pushed upward to become larger. Each breaking wave can pick up a sizable load of sand, shells, or whatever is available and move it forcefully up the beach. As waves pick up sand and move up onto a beach, some of their water soaks into the beach. Since small waves contain less water, they lose a greater percentage of their volume and thus a significant amount of their ability to move sand.
Larger waves hold greater volumes of water and thus lose a smaller percentage into the sand.
This means that large waves can hold their loads in suspension long enough to carry the sand not only onto the beach, but also back out again. The result is that large waves tend to transport material off the beaches, while smaller waves with less transporting ability tend to build beaches up. This may be seen on many beaches that become smaller during the winter when waves are large, and grow back in summer when waves are smaller. The second motion is along the beach. The careful observer will note that most waves approach the shore at some angle, not straight-on.