This is actually an interesting thread, good idea.
I think inherently, when people first get into fragrances, they aim on building a solid collection, looking up to the 'old hands' with large collections, wanting to be like them, sooner rather than later. So (i'm guessing), a lot started with 30 to 50ml bottles. Until they realized they had spent a fair amount on fragrances, and started to become more conservative, only purchasing large bottles of select fragrances.
As alcohol oxidizes in relation to the surface area in contact with the air, relative to the amount of liquid, 100ml bottles technically are a better idea for storage of a fragrance. But if one wishes to learn or build up a scent memory of a particular fragrance, one could survive on 3ml or even just 15ml. Just keep it in a cool dark spot. If you wish to learn properly by wearing it, 30ml!