Mame and the controls mapped there, even if you are using the xarcade config, have nothing to do with BigBox and the controls inside BigBox. Inside BigBox you have to map the keyboard controls and those are only for moving around inside BigBox. It does not do affect the emulator at all. Did you go into the keyboard settings inside BigBox and map your controls?
Even if you are using the xarcade config mame still sees the commands a keystrokes. It is just the config file already knows what the stock key configuration is for an Xarcade tankstick. If you look in the config you will see it lists Keycode_LALT, Keycode_SPACE, and so on.