Scale down all site in Semantic UI

Semantic UI uses Gulp to compile the CSS and Javascript for use in your project. You need the change the base font-size to make your components smaller. The thing is, you need to override this value before compiling. I'm afraid that that's why the solution of Barry127 won't work as expected, since the sizes for several components have already been calculated at this point.

Change the base size in {theme}/globals/site.variables:

/*-------------------
      Base Sizes
--------------------*/

/* This is the single variable that controls them all */
@emSize   : 14px;

/* The size of page text  */
@fontSize : 14px;

Now compile everything by running gulp build. If you don't want to use the build tools shipped with Semantic UI, use your own (custom) gulp functions. Read more about theming here.

Never up- or downscale the whole document, this is not a good practice. There are still browsers out there that don't support the transform property. Also, this may cause text and borders to appear blurry.