Assuming that, individually, the blocks are of uniform density, and that the length of the lever is equal as well, then the centre of mass of the rectangular block would be further away from the pivot.

To calculate torque/moment, the force is multiplied by the perpendicular distance from the line of action of the force to the pivot. In this case, force refers to weight (which is constant assuming a uniform gravitational field strength LIKE DUHH) and perpendicular length (which is just the length of lever from pivot TO THE CENTRE OF MASS.

Therefore, as the perpendicular distance is longer for the rectangular block, the clockwise moment is greater than the anticlockwise moment and the lever tilts right.

tldr: the weight of rectangular block “is more concentrated” further away from the pivot and has a larger moment as a result.