Trying to replace graphics resources in a PDF - PDFBox 2.0.8 - pdf

Question

I'm trying to manipulate image resources in some PDF files; the workflow is: extract image resources -> process each -> replace old ones with the new.
Simple task really, I have working code for extracting and replacing, but when I replace, the new file size is nearly twice the original.
To replace the images, I use PDResources.put(COSName, PDXObject). Any ideas what would cause the size increase in the resulting document? It happens even if I completely omit the middle step in the workflow to process each image resource.
public static void PDFBoxReplaceImages() throws Exception {
PDDocument document = PDDocument.load(new File("C:\\Users\\Markus\\workspace\\pdf-test\\book.pdf"));
PDPageTree list = document.getPages();
for (PDPage page : list) {
PDResources pdResources = page.getResources();
for (COSName c : pdResources.getXObjectNames()) {
PDXObject o = pdResources.getXObject(c);
if (o instanceof PDImageXObject) {
counter++;
String path = "C:\\Users\\Markus\\workspace\\pdf-test\\images\\"+counter+".png";
PDImageXObject newImg =
PDImageXObject.createFromFile(path, document);
pdResources.put(c, newImg);
}
}
}
document.save("C:\\Users\\Markus\\workspace\\pdf-test\\book.pdf");
}