Just use the presets as they are, with joint stereo. I use preset extreme because I can occasionally tell the difference in snare drum or cymbal sounds, and mostly because I'm bored and felt like reencoding my files. Most lossy encoders are optimized for joint stereo and I thought that it didn't change the filesize. The only way that I can think of that the file sizes would be different is if you tried to make something similar to a preset using a bunch of command line parameters or if you changed between vbr old and vbr new. Until, or if, the LAME developers change the presets to vbr new, instead of vbr old, you should stick with vbr old. I would like it if vbr new was the norm because it is a lot faster with a similar file size and quality to vbr old.