Checking out the history of Lingerie on wikipedia, the word lingerie was derived from the French word "lin" for Linen. Though the original word meant both men and womens' undergarments but in English the word Lingerie only refers to women undergarments. The sexy and erotic looking ones in particular. The material should be made of either, satin, silk, lycra, lace,nylon, polyester or nylon. Cotton women undergarments are not considered as lingerie. So it was explained in Wikipedia.
The purpose of lingerie from the time it was designed for women wear up to now has change tremendously. From the beginning of lingerie and up to the mid of 20th century, the purpose of lingerie was at first for shaping the body. Reminds me of Scarlett of Gone with the wind in the scene where she had to hold her breath when her maid tried to tie her darn tight corset. Another reason is for hygiene and of course for modesty. Modesty seems to be something long forgotten in nowadays society.
Anyway, as you can see in the picture above, womens lingeries used to be big, bulky and most probably warm. Then lingeries started to become smaller and less bulky and in the 1960's, Frederick's of Hollywood, (a Hollywood lingerie retailer) new lines of lingeries started to make lingeries as a sex appeal instead of just plain undergarments.
When lingeries started to become more and more beautifully designed in the 21st century, I believe that is when they started to have the idea of wearing lingerie as an outerwear as well instead of an undergarment only.
So the idea that undergarments became smaller is just not due to global warming. :) But more for the purpose of sex appeal.