Why my logo shows up smaller that the file I upload?

Each theme resizes the logo for design purposes, to make it fit into the layout.

To find out the proper logo size for your theme, check these articles:

Here’s how to prepare your logo file before uploading:

Logo Guidelines — How to prepare the logo file to upload

If you have a logo that must be a little bigger or smaller than what the theme proposes, visit your theme’s forum and get a CSS snippet to achieve that.