convenience store vs megastore

convenience store

noun
  • A small retail store, often in a residential area, that carries a limited selection of items such as staples, junk food, and drugstore items, and which is open long hours for the convenience of shoppers. 

megastore

noun
  • A large supermarket or similar retail outlet; a superstore. 

How often have the words convenience store and megastore occurred in a corpus of books? (source: Google Ngram Viewer )