Actually, the size image reduced when the browser page resized. On other things, the size of image will try to adjust with column (wrapper) and the number of the column will adjust depending on screen size. So in the small screen size the number of column will decrease (maybe 2 or 3 column) and the image will go below on each other.
At first, the size of image will reduced and try to adjust with the size of column (wrapper). If the screen size is too small, the column will go below on each other and the image won’t be reduce their size.