convert a position from a smaller image to a bigger image.

edit

You can transform points from the small image to the big image (or viceversa) by a simple cross-multiplication (rule of three). For example:

    cv::Mat smallImg, bigImg;
    cv::Point2f bigPnt, smallPnt;
    ...
    bigPnt.x = smallPnt.x * float(bigImg.cols) / float(smallImg.cols);
    bigPnt.y = smallPnt.y * float(bigImg.rows) / float(smallImg.rows);

A rectangle (ROI) can be transformed in the same way:

    cv::Mat smallImg, bigImg;
    cv::Rect bigROI, smallROI;
    ...
    bigROI.x = smallROI.x * float(bigImg.cols) / float(smallImg.cols);
    bigROI.y = smallROI.y * float(bigImg.rows) / float(smallImg.rows);
    bigROI.width = smallROI.width * float(bigImg.cols) / float(smallImg.cols);
    bigROI.height = smallROI.height * float(bigImg.rows) / float(smallImg.rows);