Logo too small on login page

I'm trying to make the logo larger on the main logon page. In OC v8, the identical svg file appeared quite large above the logon box (how I wanted it), but in 9.1, the same logo appears way smaller. I think it might be something to do with the 'viewbox' boundary assigned for the logo, so it keeps scaling the logo down. I've had a look at various files, but can't see where this is defined.