Making a theme smaller?

Not really, robust modern Browsers do this not to make them smaller (really) but bigger for those with readability issues (try 400% on a good screen). That said, you can make many things have less padding, margins, smaller fonts, make the site ‘display in a browser’ smaller (by default) with custom CSS work:

To discover what CSS is output by your theme, use a web inspection tool such as Firebug: https://getfirebug.com/ , Chrome Inspect Element: https://developers.google.com/chrome-developer-tools/ or Internet Explorer 9+ F12 Dev. Tools: https://msdn.microsoft.com/en-us/library/dd565627(v=vs.85).aspx#csstool

There are others.

When editing CSS, use a Child Theme
https://codex.www.ads-software.com/Child_Themes
Custom CSS Plugin, or Theme provided custom CSS option.
Edits to parent themes are lost on theme update.

Learn CSS: https://www.w3schools.com/css/

Thanks, the theme is one I made myself, was just looking for an easy way to make it smaller. Thanks thow!

I guess using javascript to change all values in your css to 75% of there original value is out of the question?