In almost every respect, the Arduino Mega 2560 is identical to its smaller sibling. It runs at the same speed of 16MHz, requires approximately the same amount of power, executes the same software, and uses the same development tools.
The primary difference between the Uno and the Mega is the processor, the ATmega2560, which has more memory and more peripherals than the ATmega328. The PCB is also larger, but it maintains form-factor compatibility with the standard Arduino, adding three additional expansion connectors along the right side and extending the PCB by approximately an inch in length. The remainder of the circuit is essentially identical to the Arduino Uno. See Figure 1-11.