Why are some resisitors physically larger than others?

Question

The resistors I'm saying is the 'carbon composition resistors'.

Answer 1

The larger the resistor the larger the surface area to dissipate heat. If a resistor is to be used in a high power circuit it needs to be larger so that it does not get too hot.

Answer 2

Wattage. Larger resistors can dissipate more heat.

The material used has some influence too - precision metal film are usually a bit larger watt-for-watt vs. carbon.