This chart shows the capacitance characteristics when voltage is applied. When voltage is applied to the tantalum capacitor, capacitance does not change. However, when voltage is applied to the ceramic capacitor, capacitance decreases greatly. For example, when a ceramic capacitor of 6.3V 22µF, 2012 size is used in a 3V line, actual capacitance will be 10µF. If it is replaced with a tantalum capacitor, the same capacitance is obtained by using a 6.3V 10µF, 1005 size, which is two times smaller in size. Another example could be, when a  6.3V item is used and 70% capacitance of rated value is needed, a ceramic capacitor can meet up to 1.8V. Usually a tantalum capacitor needs voltage derating for safety, but a tantalum capacitor can be used up to the 3.3V or 4.2V values tested in this graph.