Why does the same grade of yarn sometimes vary in thickness?

You may have noticed that bizarrely sometimes the same grade of yarn can seem thinner or thicker according to the colour. Now 2 ply should be 2 ply if it is the same composition from the same manufacturer, but I always find that the big cones I buy can be quite a different size for the same weight, for example a white cone is quite small but a dark pink one can be half as big again. Some colours will wind into quite a small skein, but the same weight in a different colour may give a much more bulky skein. The answer to this conundrum is that the dyes used can affect the yarn, making it more or less bulky, by causing the fibres to swell. The same effect is often seen with embroidery silks; some users will swear that the darker colours are thinner. The good news is that for miniature knitting it does not really affect the tension, and hence the finished size of the item. So if you have noticed this, yes, you have not imagined it, but you don't need to worry about it!