The word 'pond' originally meant much the same as we now mean when we say 'pound' as in 'dog-pound'...ie an enclosed space. In other words, the earliest ponds were artificial and usually created for a specific purpose - mill-pond, skating-pond etc. Over time and in certain areas, the word came to mean any small body of water, whether natural or artificial. Given that they were basically man-made to start with, they were obviously of relatively small size compared with, say Lake Michigan! On the other hand, Brits and Yanks often refer - albeit jokingly - to the Atlantic Ocean as 'the Pond'!