The most common query which we hear from numerous people is why the freak jeans and other cotton made outfits shrink! Possibility is very high that the maximum is during the first wash.I understand we follow all the guidelines minutely as stated, but still we are in turmoil and trouble. The root commences during the manufacturing process, especially the fabric. Cotton thread used in fabric weaving is spun to high counts to give it strength and fine neat look. When this fabric is first inserted into water, the cotton swells and gets compact due to those thread twists. This phenomenon increases the quantity of cotton per square inch of fabric and reduces the length and breadth. 
The same scenario is with the cotton jeans with an additional factor inculcated that is desizing of jeans. When denim is made, its warp thread is sized to avoid its breakage during the weaving process. This warp thread is starch coated which gives a robust and rough character to denim. The starch is water soluble and washed off in simple rinse and when this starch is removed the fabric comes back to its relaxed position and shrinks in all proportions. This joint effect of cottons natural swelling and desizing collectively gives extra shrinkage in denim jeans and gives special character to this outfit. 
As we all know,jeans come in multiple combinations. The fact is that Cotton polyester blended jeans shrink less as compared to pure cotton jeans. If you ever noticed, this fabric combination is always mentioned in labels inside jeans. Never forget to look at that when purchasing your jeans. Secondly, in stretch denim, elastane is mixed with cotton to give stretchy character and here the shrinkage is important, in fact it is mounted as per our body size and weight. 
I believe you have a basic overview and understanding that jeans are preshrunk before. We should be grateful to jeans manufacturers, the jeans that you find in the market are pre-shrunk and contain its specifications on label and we can buy accordingly with an overview. After washing it might shrink to some centimeters but after you wear it for a long time you will find it coming back to first fit! 
The same scenario is with the cotton jeans with an additional factor inculcated that is desizing of jeans. When denim is made, its warp thread is sized to avoid its breakage during the weaving process. This warp thread is starch coated which gives a robust and rough character to denim. The starch is water soluble and washed off in simple rinse and when this starch is removed the fabric comes back to its relaxed position and shrinks in all proportions. This joint effect of cottons natural swelling and desizing collectively gives extra shrinkage in denim jeans and gives special character to this outfit. As we all know,jeans come in multiple combinations. The fact is that Cotton polyester blended jeans shrink less as compared to pure cotton jeans. If you ever noticed, this fabric combination is always mentioned in labels inside jeans. Never forget to look at that when purchasing your jeans. Secondly, in stretch denim, elastane is mixed with cotton to give stretchy character and here the shrinkage is important, in fact it is mounted as per our body size and weight. I believe you have a basic overview and understanding that jeans are preshrunk before. We should be grateful to jeans manufacturers, the jeans that you find in the market are pre-shrunk and contain its specifications on label and we can buy accordingly with an overview. After washing it might shrink to some centimeters but after you wear it for a long time you will find it coming back to first fit!