There is no 16 bit or 32 bit mp3 format. The mp3 is fully independent from this.
However, you might see that WaveLab decode a files in 32 bit. But this is independent from the mp3 contents itself.
I think your problem is elsewhere. Maybe you use VBR while you should use CBR, or the inverse. Try this.
You can also try the Fraunhofer encoder vs the Lame encoder.