Why are some AC/DC adapters light while others are heavy?
A "heavy" power supply contains a 50/60 Hz transformer to provide the necessary isolation between the mains and the DC output. This transformer has a laminated steel core that needs to be fairly massive in order to provide an adequate impedance to the line voltage.

A "light" power supply uses active electronics to convert the line frequency to a much higher frequency (on the order of 20 to 100 kHz). This allows the isolation to be achieved with a much smaller transformer that has a powdered ferrite core.

As long as the output ratings are compatible (same voltage, same or higher current rating), they're mostly interchangeable. Beware of the difference between regulated and unregulated power supplies.