Apple - Why does Numbers hide a huge PNG file in exported Excel sheets?

I'm not sure why it's included in the export, but this is one of the default "Image Fills":

tl;dr: it's the default shape fill, strictly unnecessary if there are no shapes.

XLSX, DOCX and other formats use OPC (the open packaging conventions), which mandates the zip container and describes how files should be laid out. If there is a file that you do not recognize, look in the various relationship files (they end in .rels).

In this case, the relevant line can be found in the themes relationship file xl/theme/_rels/theme1.xml.rels:

  <Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/image" Target="../media/image1.png"/>

The file will be referenced in xl/theme/theme1.xml as rId1. Using ECMA-376 as a guide, you will find it referenced as the default shape fill:

<a:objectDefaults>                    <-- shape/line/text defaults
  <a:spDef>                           <-- shape defaults
    <a:spPr>                          <-- shape properties
      <a:blipFill rotWithShape="1">   <-- picture fill
        <a:blip r:embed="rId1"/>      <-- references the picture

The writer must not be omitting the image if there is no shape.

Keynote files exported to PowerPoint do things like this too. That PNG is a background fill from Keynote, and is made available to PowerPoint to be used as the default shape fill for shapes created in the exported document. Keynote not only exports your document, but also the template style elements it was created with - even if some of those style elements (i.e. the background image) haven't been used yet.