I am working on a big house drawing with the dimension of 12.00 x 36.00 meters.

My layout to print is A3. To fit the floorplan I would need to have scale 1:100. This means my floorplan is measured 12.00 cm x 36.00 centimeters on the sheet.

I put my A3 layout, add my viewport. Then I select my viewport and select the scale 1:100 in the drop down menu. Now the drawing is tiny! And it should be bigger. 

I measure in my drawing and it says 12 x 36 meters. What I am doing wrong? 

I am working on a mac. 

Please help, thank you