Not only will a flat brush create a broad brushstroke, but if you turn it so you're leading with the narrow edge, it'll produce thin brushstrokes. A short flat brush is ideal for small, precise brushmarks.
A flat brush's paint carrying capacity is determined by the bristles it has, and by the length of these. A short-haired, synthetic-bristle flat brush will hold less paint than a long-haired, mixed or natural-hair brush. The flat brush in the photo has got hog hair, which holds paint well and, being stiff, is ideal for leaving brushmarks in paint should you wish to do this.