Justoverclock when you upload a logo to Flarum, Flarum resizes the file to be 60px tall.
The file is then used in the header, where Flarum CSS force it to be 30px tall, effectively giving it a 2x pixel density.
In your situation, you have some padding inside of the logo itself, so I would recommend cropping the image down before uploading it to Flarum.
If that's still too small for your liking, you could tweak the CSS of the header, but making the header taller could lead to issues with Flarum responsiveness.
