Duplicated pdf larger than original
The size difference of a few hundred kilobytes can be attributed to the PDF version and encoding.

Recent versions of PDF introduced features to reduce the file size. These features included compressed object streams in v1.4.

A PDF file can look identical but be stored on disk in nearly infinitely different forms. The application responsible for creating the PDF is critical to getting the smallest possible file size.

Most applications, such as Preview, will call out to a general purpose PDF framework or library to deal with the file format. This will result in well formed files with minimal effort by the application's developers.

Other applications may create PDFs highly-tuned to their content and needs. The PDF specification allows for creative reuse of content, de-duplication, and optimisations that require knowledge of the document's purpose and intention. This requires significant effort and is increasingly rare.