"A dig" (noun) a place where people dig or have dug (verb), maybe to build something or an archaeological site. I googled it and this phrase is from a book for very small children. It is really just that the words "big", "pig" and "dig" rhyme and it is used to teach kids the sounds. So, it doesn't really have a meaning, it is just a way to use three words that have the same sound. Hope that makes sense, Molly.