Nicki, Yes different size breasts are quite normal, and likewise different
milk volumes from each breast is also normal.  Peter Hartmann has published
his work on this demonstrating that it is quite normal and the volumes can
vary considerably as you've pointed out.  His latest work is quite
fascinating following a group of women through from pre-conception (he did
mention that recruitment wasn't easy!) to several months post partum.  One
woman had NO breast changes whatsoever during her pregnancy - well I thought
I knew what that meant! Anyway she went on to breastfeed quite successfully.
See if you can find his recent articles on milk synthesis at your local
uni/hospital/whatever.
Denise Fisher, Brisbane, Australia