I think. It is due to Genes.

I think there may be solid reason for this like eating ,activity,environment etc effect.

May be, But Genes I think are the main reason

There is also a roll of nutrition along with Genes for our body growth there was good diet in earlier times and all diet was pure and today's diet is adulterated. So I think this could also be a big reason for decreasing height.

It's not JEANS but GENES. I am sure, only Genes are responsible for the decrease in Size.

Thank you very much for showing me my mistake. I just improve it, by mistake it happened.

it's OK.

Peoples are decreasing in size day by day as compare to long years? reason..

Not sure from where you got this  idea? Is there any statistics to prove it or any scientific research paper published on the subject ? I have never come across any so far except the fact that size of the human brain is decreasing ..

I am not aware of this fact but I feel if at all size of human beings is decreasing gradually then, one main reason is that children are not regularly playing outdoor games. I stay in Delhi and I can see the upcoming generation having good height. Here, swimming, cycling, and other sports are very popular among the children and naturally, their growth is very good. 

The topic is misleading.