The size of the logo image is fixed.
If you want to use big size logo image you need to define logo image size in functions.php file
Go to Appearance -> Editor -> functions.php
at line no. 40 you will find the below code, just change the logo image height and width that you want to use
add_theme_support( 'custom-logo', array( 'height' => 100, 'width' => 100, 'flex-height' => true, ) );