Can't resize image with add_image_size

Hello all,

I added add_image_size in functions.php and its resizeing the images when the image is large enough, but when the image is smaller than the size I want its not resizing the image.

Any idea on how to resize small image to large size.

The Code:

if ( function_exists( 'add_image_size' ) ) {
	add_image_size( 'category-first', 255, 150, true ); //300 pixels wide (and unlimited height)
	add_image_size( 'category-next', 210, 100, true ); //(cropped)
	add_image_size( 'slider', 250, 250, true ); //(cropped)
}

Thanks