This is closely related to whether you store a bitmap or an appropriate number of pictures. If your text and pictures are stored in bitmap mode, this font will be obviously blurred or jagged when enlarged, and the compression rate of bitmap files is relatively low, so the generated files will be very large. If stored in a vector image, the text is still clear when it is enlarged and reduced. Because it is stored by a mathematical algorithm, the file size is usually small.

100M file is probably a pdf generated by scanning documents, and 16M file is a pdf generated by electronic documents.