why file upload change photo size php code example

Example 1: php.ini upload image size

##put these directives in a php.ini file in the root of files
upload_max_filesize = 10M
post_max_size = 10M

Example 2: php resize image on upload

$maxDim = 800;
$file_name = $_FILES['myFile']['tmp_name'];
list($width, $height, $type, $attr) = getimagesize( $file_name );
if ( $width > $maxDim || $height > $maxDim ) {
    $target_filename = $file_name;
    $ratio = $width/$height;
    if( $ratio > 1) {
        $new_width = $maxDim;
        $new_height = $maxDim/$ratio;
    } else {
        $new_width = $maxDim*$ratio;
        $new_height = $maxDim;
    }
    $src = imagecreatefromstring( file_get_contents( $file_name ) );
    $dst = imagecreatetruecolor( $new_width, $new_height );
    imagecopyresampled( $dst, $src, 0, 0, 0, 0, $new_width, $new_height, $width, $height );
    imagedestroy( $src );
    imagepng( $dst, $target_filename ); // adjust format as needed
    imagedestroy( $dst );
}