Here are three clones of the Arduino MEGA 2560 much more compact built around a microcontroller ATmega2560-16AU. The ATmega2560-16AU is an 8-bit micro-controller which has 256kB of flash memory and a 4kB EEPROM which allows to store configuration parameters for example.

Here are the main features of the MCU

Flash Program Memory: 256 KB of which 8 KB are used by the bootloader
SRAM memory (volatile memory): 8 KB
EEPROM memory (non-volatile memory): 4 KB
Clock speed: 16 MHz
Digital I / O pins: 54, including 14 PWM
Analog input pins: 16, can also be used as digital I / O pin
Max. Current available per I / O pin (5V): 40 mA (200 mA cumulative for all I / O pins)
Max. Current available for 3.3V output: 50 mA
Maximum current available for 5V output: 500 mA (depends on the power supply)
Operating voltage: 5V
Recommended supply voltage: 7-12V
Supply voltage (limits); 6-20V

Three models of Arduino Mega 2560 Mini

RobotDyn offers two models

Arduino Mega 2560 Pro with integrated USB / UART interface (55mm)
Arduino Mega 2560 Pro Mini without USB / UAR interface, a little more compact (52mm)

Here is a comparison to the original version.

The Arduino MEGA 2560 mini rev3 is based on the ATmega2560 micro controller more powerful than the Arduino Uno as its brother. It has 54E / S (15 of which are PWM), 4 serial ports (UART) and 16 analog inputs. The pinout is as follows

The second manufacturer to offer a compact version is Inhaos who has also developed a perforated plate prototyping board with integrated power supply.

Arduino Mega 2560 Pro and RobotDyn Mini

The original Arduino Mega is not very compact. The Arduino Mega 2560 Pro developed by RobotDyn measures only 55 x 38 mm. The Pro Mini version is even more compact. As it does not ship a USB / UART converter, the board is only 38x52mm.

There are 6 variants:

Pro 5V with header / without header
Pro Mini 5V with header / without header
Pro Mini 3V3 with header / without header

Arduino Mega 2560 Mini from manufacturer Inhaos

This compact version developed by Chinese manufacturer Inhaos is available with or without USB / UART cable. Here is the pin spotting

Prototyping support from the manufacturer Inhaos

Inhaos also offers an EX-2560LT reference development platform on which the Arduino Mega 2560 Mini is stacked. It incorporates a 5V power supply and a perforated plate for tuning circuits. It’s a little less convenient than a breadboard to start. The usable area for the circuit is 100x200mm. The perforated plate is double sided. A selector is used to select the supply voltage between 5V and 3.3V. The board can be powered using a DCIN connector with a voltage ranging from 9 to 24V.

This prototyping board has a female connector compatible with the Inhaos Arduino Mega 25600 Pro Mini (not compatible with RobotDyn boards).