It's a common misconception that VBR is supposed to produce smaller files. Yes it will on certain types of music, but that's a side effect. The point is to allow the encoder to exceed a predetermined kbps when the need arises, thus ensuring quality is maintained (within the confines of MP3 lossy encoding). If you force it stick to a max. level using CBR you can get distortion. E.g. Ray of Light (Madonna) will highlight this when partway through the track just before a chorus there's a rapid scale spanning a couple of octaves using a high energy electronic sound (even minidisc using coaxial digital link can't cope with that). ABR is a more conservative form of VBR, so the kbps changes within in a smaller band of settings, thus it won't differ in size to a CBR encoded file.

An MP3 encoded using VBR will sound better than an ABR which will also sound better than a CBR mp3.