Over VERY long timescales (millions and millions of years), erosion can reduce a volcano's size a LOT. A great example of this is the Hawaiian-Emperor seamount chain. The Hawaiian-Emperor seamount chain is a line of volcanic islands that has been made by a hotspot that sits still underneath the Pacific plate as the plate moves above it. The Hawaiian-Emperor seamount chain starts at Loihi (the newest Hawaiian volcano, which is still underwater), and ends just south of Alaska at a place called the Aleutian Trench. I've included a map that shows the locations and shapes of each volcano, as well as their ages (each number is an age in millions of years).

Although each volcano may not have started out the same size, over time (due to the erosive forces of wind and water) the Hawaiian Islands that exist today will erode until they are no longer above the ocean (but don't worry, because it's millions of years off into the future). Instead, they will be seamounts, just like the rest of the Hawaiian-Emperor chain. In fact, that is how the seamounts in the rest of the chain formed!