Anything described as large-scale is happening in a big way. A large-scale study of the benefits of dark chocolate would require many people and lots of time. And lots of chocolate. Yum.
Scale has to do with size, so anything large-scale is huge. A large-scale ad campaign goes all around the country — and maybe around the world. A large-scale military campaign is one that might attack on multiple fronts. Cartoon villains who want to take over the world commit large-scale crimes. A large-scale commercial farm is gigantic, cranking out tons of food. The opposite of large-scale is small-scale.