Think of an ESC like wiring. You can use heavy duty cables to carry light loads, but put some small wire on a large load and watch the fireworks.

A more capable ESC can handle lighter loads.

As far as how to determine loads, you can go to the GWS site and browse their tables. Start here.

http://www.gws.com.tw/english/produc...owersystem.htm

You can use a online program like P-calc too.

http://brantuas.com/ezcalc/dma1.asp