CascadeClassifier::detectMultiScale alogical influence of minSize parameter

I work with OpenCV 2.4.9. When I downsize whole image with, lets say, `k=0.6` koefficient (both width and height reduced proportionally), I do the same with parameter `minSize=minSize(minSize_original_width x k, minSize_original_height x k)`, the same with `maxSize`. I visualize detecting result. The detection rate of bigger objects stay the same in reduced image, but smaller objects are not detected anymore as they were in the original, larger image. Is there any other, hidden constraint, which I don't know? I use pre-trained model.