Safe minification
Many minifiers collapse every run of whitespace. That is fine for element-only data but corrupts documents with prose: “Leave with <em>reception</em> if nobody answers. Fragile!” loses its double space and the gate-code block under xml:space="preserve" loses its layout. BeautifyXML removes whitespace only where it can show the whitespace was formatting.
The size saving is shown in the status bar, and the Changes tab shows exactly which lines went away.
Removing comments
Comments can carry licence notes or processing hints, so removing them is a separate, opt-in choice. Comments inside protected regions are kept even then, and the number removed is reported.
Questions
Does minifying change empty elements?
Only if you choose an empty-element style. <a></a> and <a/> are the same element to an XML parser, but the change is reported because signatures and byte comparisons notice it.
Is the minified file still valid?
It is always re-parsed and compared with your source before it is offered. Schema validity is a separate question that BeautifyXML does not check.