How much does a life sized LEGO person sculpture weigh?
The image posted in your question shows the Lego brick artist Nathan Sawaya.

On his website, he states:


  A life-size human form sculpture typically has 15,000-25,000 bricks.


So, lets use 20,000 for our calculations.

Next, how much does a brick weigh, well of course it depends on the type of brick, but if we take the common 2 x 4 Brick , Bricklink tells us that one 2 x 4 Brick weighs 2.32g.

20,000 x 2.32 = 46,400g 
= 46.4Kg
= 102.3Lb 
= 7.3 Stone


So one could say they are very roughly equivalent in weight.

But if anything, the Lego versions are likely lighter than their human twins. This is especially true if we consider that many of the bricks will be smaller than the 2 x 4 Brick we used in the rough calculation above. For example, according to Bricklink, the 2 x 3 Brick weighs 1.92g and the 2 x 2 Brick weighs 1.21g.