Change the filename extension to one that indicates a lower bit rate codec.
I’m assuming you mean wav, not WAV, as .wav, in Asterisk, uses GSM, rather than the 16 bit, signed linear, that you get with wav, and you are not going to get anything much smaller than that, with an acceptable quality. (Microsoft .wav is a wrapper format, and can be used for many different encoding schemes, but Asterisk only uses it for two.)