Saving to image with format extension create a huge file

Question

I work on tool for detecting objects in pdf files. One of the first steps is a conversion of pdf file to image file.

Here's my original code which reads pdf from file path, puts the content into Image / Bitmap object, extracts just a specific area of the image and saves extracted area into new file.

string pathToSave = fileName.Replace(".pdf", "");
PdfDocument doc = new PdfDocument();
doc.LoadFromFile(fileName);
Image emf = (Bitmap)doc.SaveAsImage(0, Spire.Pdf.Graphics.PdfImageType.Metafile, 1200, 1200);

Rectangle r = new Rectangle(13555, 615, 5860, 7240);
Bitmap target = new Bitmap(r.Width, r.Height);

using (Graphics g = Graphics.FromImage(target))
{
    g.DrawImage(emf, new Rectangle(0, 0, target.Width, target.Height), r, GraphicsUnit.Pixel);

    target.Save(pathToSave + ".bmp");
                
}

What I've noticed is that saving Bitmap to file with passing an extension format creates a file of much bigger size than without passing an extension format.

Saving file in this way results in file of size ~900kb

target.Save(pathToSave + ".bmp");

Saving file in this way results in file of size ~165MB.

target.Save(pathToSave + ".bmp", System.Drawing.Imaging.ImageFormat.Bmp);

Both files are of proper format and can be further used with the same results by Tesseract. Why there is so huge difference between those two ways of saving files?