speexenc/speexdec doubles file size

this is not really a development question, but i didn't find another
mailing list to ask it.

any idea why size of wav file doubles when it is encoded to speex and
back to wav:

$ ls -ls testi.wav 
40 -rw-r--r-- 1 foo foo 40674 May  4 14:38 testi.wav

$ speexenc --denoise --agc --quality 10 testi.wav testi.spx
Encoding 8000 Hz audio using narrowband mode (mono)

$ ls -ls testi.spx
20 -rw-r--r-- 1 foo foo 16405 May  4 14:46 testi.spx

$ speexdec --mono --rate 8000 testi.spx testi.wav
Decoding 8000 Hz audio using narrowband mode (mono)
Encoded with Speex 1.2rc1

$ ls -ls testi.wav 
84 -rw-r--r-- 1 foo foo 81464 May  4 14:48 testi.wav

-- juha