They are steep-sided because the hot spatter blobs are able to stick to each other when they land, and don't flow or roll away. The fountaining associated with the formation of spatter ramparts is usually less than 10 m high. At the end of the eruption, lava often drains back into the fissure, forming prominent drainback features . Even if nobody actually witnessed a particular eruption, if you find spatter ramparts or cones associated with it, you can say that the fountaining that formed the cone or rampart was not very high. As their name suggests, cinder cones consist of cinders, more properly called scoria.
Scoria is very vesicular, low density basalt.
The Pu'u 'O'o fountains were at times up to 350 m high, and those during the early stages of the Mauna Ulu eruption were up to 500 m high. Because the pyroclasts are thrown so high, they cool before they land and don't stick together. In plan view, cinder cones tend to be roughly circular . They are usually formed later in an eruption when activity has localized to one or more discrete vents. If the precise location of the vent changes during an eruption, the cone loses its simple circular shape, and becomes more complex.