Step 1: Recall thermal expansion.
When a rod is heated, it grows longer. The increase in length is \[ \Delta L = \alpha L_0 \Delta T \] where $\alpha$ is the coefficient of linear expansion, $L_0$ is the starting length, and $\Delta T$ is the temperature rise.
Step 2: Note what is shared.
Both rods start with the same length $L_0$ and are heated by the same $\Delta T$. So the only difference is the value of $\alpha$ for each metal.
Step 3: Write each expansion.
For copper, \[ \Delta L_{Cu} = \alpha_{Cu} L_0 \Delta T \] For iron, \[ \Delta L_{Fe} = \alpha_{Fe} L_0 \Delta T \]
Step 4: Compare using the given fact.
We are told copper has a larger coefficient, so $\alpha_{Cu} > \alpha_{Fe}$. Since the other parts are equal, the bigger $\alpha$ wins. \[ \Delta L_{Cu} > \Delta L_{Fe} \]
Step 5: Note mass does not matter.
The formula has no mass in it. Expansion depends on length, temperature change, and material, not on how heavy the rod is.
Step 6: State the answer.
Copper expands more than iron. \[ \boxed{\text{Copper expands more than iron}} \]