There is 'global.css' at ....\prestashop\themes\default-boostrap\css\global.css
Line 703
.col-sm-4, header .row #header_logo {
width: 33.33333%; }
The logo is 'forced' to occupy one third of the total space of the page. If you remove 'width:33.33333%', the logo will take on the size of the original image.
If you are going to look at this file, make sure you look at a backup copy. I don't think it is advisable to make too many changes here as things may be affected during upgrades.
Once again, backup global.css!
I have issues with this myself. I want to have a larger logo and position it where I want but there's a lack of information out there...