Use thumbnail settings as minimum size

thumbnail_minimum_size.php

add_filter( 'wp_constrain_dimensions', 'mcoc_constrain_dimensions', 11, 5 );
function mcoc_constrain_dimensions( $dimensions, $current_width, $current_height, $max_width, $max_height ) {
	// Stop if desired size is not post thumbnail
	if ( ( $max_width != get_option( 'thumbnail_size_w' ) ) || ( $max_height != get_option( 'thumbnail_size_h' ) ) ) return $dimensions;
	// Ensure cropped width is at least post thumbnail width
	if ( $dimensions[0] >= $max_width ) return $dimensions;
	if ( $current_height > $max_height ) {
		return [
			$max_width,
			$max_width * $current_height  / $current_width
		];
	}
	return $dimensions;
}