A ‘normal’ enlargement casts a ‘shadow’ of the shape, which may be larger or smaller than that shape to be enlarged. The scale factor k is positive, with the enlarged shape being k times as far from the centre of enlargement as the original shape. If the scale factor is more than one, the enlarged shape is bigger. If the scale factor is less than one, the enlarged shape is smaller.
If the scale factor is negative, then the enlarged shape is more like a ‘mirror image’ than a shadow, on the ‘opposite side’ of the centre of enlargement.