this is the way I understand matching a transformer to a device…
With say a input 100/240v 50-60hz 0.5A transformer You want to match the transformer output voltage to the device exactly, and with amps it is better to go over what the device needs, then it is to go below what the device needs. (Volts are like the water that runs through a pipe, where amps are like the size of the water pipe itself…larger piping is ok, smaller piping is not. You also do not want the water pressure to be greater than the pipe can handle, and you do not want a larger pipe with greater pressure than the outlet fittings can handle).
For example, let’s say my device requires 10W (5V 2A), It is ok to use a transformer at 5v 2.5A, it is NOT ok to use a transformer at 6V 700mA (.7A). It is NOT ok to use a transformer at 10V 3A.