How to Enlarge each rectangle (boundingbox) from findcontour Opencv C++ - c++

Question

I have a program to detect an Image using bounding box. But, somehow only the center of the image (which have a high saturation) can detect. I want to enlarge the bounding box so the object with low saturation can be detect too.Here is the image image result
The red rectangle is the result of boundingbox from findcontour. I want the rectangle like the blue one, so the large object can be detected. Here is my code:
#include "stdafx.h"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
using namespace cv;
using namespace std;
int main(){
Mat frame = imread("32.jpg", 1);
Mat mask(frame.size(), CV_8UC1, Scalar::all(0));
Mat hsv;
int a = 5;
cv::cvtColor(frame, hsv, CV_BGR2HSV);
std::vector<cv::Mat> channels;
split(hsv, channels);
cv::Mat H = channels[0];
cv::Mat S = channels[1];
cv::Mat V = channels[2];
Mat th;
threshold(S, th, 90, 255, THRESH_OTSU);
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
/// Find contours
findContours(th, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_NONE, Point(0, 0));
vector<Rect> rekt(contours.size());
/// Draw contours
Mat drawing = Mat::zeros(th.size(), CV_8UC3);
for (int i = 0; i< contours.size(); i++)
{
if (cv::contourArea(contours[i]) < 500) continue;
rekt[i] = boundingRect(Mat(contours[i]));
rectangle(mask, rekt[i].tl(), rekt[i].br(), Scalar(0, 0, 255), 2, 8, 0);
}
imshow("ori", frame);
imshow("olaaa", resImage);
waitKey(0);
}
I have tried the other code such as here and this here but the image doesn't show the rectangle and only show black background.
Thankyou.