As someone who knits and crochets, I have noticed a difference in the thickness of Red Heart yarn.
GeriB is onto something when she mentions that different colors affect things. Anyone who paints knows that different pigments have different viscosities. And anyyone who does any kind of needlework knows that some yarns or threads are thinner than others.
So I discussed this with my husband who is a plastics process engineer. He has been in the business globally for many years and his background is diverse, although in the last 15 years, he has been involved with medical and pharmaceutical plastics. He said that there are several variables that would affect how thin the yarn is. One would be the chemical structure of the plastic being spun into yarn and the pigment would play a part in the thickness of the plastic. How much liquid was used in the mix would matter. Very importantly would be how fast the machines were running and thus how fast the plastic was being spun into yarn. The faster the machines, the thinner the thread. The thinner the liquid, the thinner the yarn. The gauge of the yarn (for example, bulky weight) would have an industry standard high or low normal that it could fall between, and that is why some yarn lots might seem to be slightly thinner than others. This could very well be on purpose to get more product out the door, or it could be because two different factories are producing the same yarn and one is following the high normal standard, while the other is following the low normal standard.
Also, since the recession hit, we have all seen companies downsizing their packaging and additionally the protocol involving quality standards of what is acceptable has changed. This is one of the many reasons I like purchasing second hand stuff.