Clothes of size S shortage

As much as I would like to claim that I am indifferent to opinions of others, but that is not the case. With this, I realised that I only have  one jumper that I don't mind wearing to university. That generally translate to; I don't wash my jumper for a long time. So recently, I was on a search for my next jumper. 

How hard is it to find a decent jumper that fit me? Very hard indeed. Ok, my size is S and occasionally M, but I noticed that size M is getting larger and larger. Similarly, size S seems slightly too big for me now too. That is beside the point. As a thrifty boy, I don't want to pay more than $30 for my jumper. So, I went on a bargain at different sale. What I found was that there simply no (next to nothing) size S jumper. Similarly, size M was hard to come by. When they came, generally, they were bad designs. All I found were sizes XXL, XL and L. It seems that Melburnians have been "upsize", even during the recession. 

On the other hand, it could be that everyone has already taken most of size S and M. So, there weren't many left to sell. However, that begs another question: If most people wears size S or M, then why the manufacturers keep producing too much L, XL and XXL. Further, it is reasonable to assume that human population size follows - to certain degree - a normal distribution. So, why don't I see more XS and XXS in the shops?

Anyone with answer, please tell me.