All LibreOffice modules use XML as its native file format. Math module uses a package structure a little different, but some xml inside the package: MathML is an XML standard for formulas. Also, some configuration files use XML. These files structure will always be there:
meta.xmlcontains information about the document (author, date of last backup).
styles.xmlcontains the styles used in the document
Content.xmlcontains the main document content (text, pictures, graphics ...)
Settings.xml, usually specific to an application, contains some parameters such as the selected printer...
META-INF/manifest.xmlcontains additional information about other files (such as the MIME type or encryption).
The images are saved in the native format inside the zip. Now you understand why LibreOffice files take up so little space on your hard disk!