Why is my file larger after a PDF split/merge operation; and how can I fix the issue?

Q: I have an issue with splitting PDF files. The file size of splitted
PDF is occupying more size than the actual PDF which was splitted.
Following is the sample of code what I have used. There is no issue
with splitting but just size is the issue. I have attached sample
PDFs. One with all 5 pages and 1 with single page which is splitted
document. Following is the code I used:

            PDFDoc doc = new PDFDoc(sourceFilePath);
       TextExtractor txt = new TextExtractor();
                PDFDoc pdfDoc=null;
                for (int pageCount =1; (pageCount <=
                    pdfDoc = new PDFDoc();
                    pdfDoc.Save(directory.FullName + @"\Tmp" +
pageCount.ToString() + ".pdf", 0);
                    pageCount += 1;
A: To reduce the file size you need to use pdfdoc.ImportPages()
before PagePushBack().

For an example of how to use this function, please see the last code
snippet in PDFPage sample: