Rustic Cottage Gardens have a softness which creates a sense of movement that more structured gardens lack. In small gardens the use of flowing curves will make the garden feel much bigger.
In nature there are few naturally occurring shapes that are completely angular, we are naturally more inclined to be drawn to curves and circles. Think of valleys, streams and the contours of hills, where nature slowly creates smooth flowing shapes.
The maintenance of a curved garden is something to consider, as a curved garden will have wider borders than straight borders.