Imagine they were 2 squares. Area of smaller one is 2 incehes squared and Area of bigger one would be 4 inces squared. So side of the bigger square would be 2 and the sides of the smaller square would be root 2. Same principle but with triangles.
Or you can say it this way. If you increase the dimensions of a triangle by a factor of 2 then the area increases by a factor of 4. Therefore to increase the area by a factor of 2 you must increase the dimensions by a factor of root 2. coz root 2 squared is 2.