diff --git a/docs/install.md b/docs/install.md
index 4dc94d7d..4536e7fc 100644
--- a/docs/install.md
+++ b/docs/install.md
@@ -83,6 +83,9 @@ cd /usr/ports/graphics/vips/ && make install clean
 
 ### Heroku
 
+libvips and its dependencies are fetched and stored within `node_modules\sharp\vendor` during `npm install`.
+This involves an automated HTTPS download of approximately 6.5MB.
+
 Set [NODE_MODULES_CACHE](https://devcenter.heroku.com/articles/nodejs-support#cache-behavior)
 to `false` when using the `yarn` package manager.