Check WordPress size on disk

Why using a plugin for that?

You can set a function to calculate the size of the 2 installations. The size of abc.com will include also the size of the other installation, but then you can subtract the smallest from the largest.

$path = ABSPATH;
$size = foldersize($path);


function folderSize ($dir)
{
    $size = 0;
    foreach (glob(rtrim($dir, "https://wordpress.stackexchange.com/").'/*', GLOB_NOSORT) as $each) {
        $size += is_file($each) ? filesize($each) : folderSize($each);
    }
    return $size;
}