Originally Posted by A and C
A thin object will (all other factors being the same) pick up ice faster than a thicker object.
and
Originally Posted by Fuji Abound
Yes, but doesn't ice start to form far more quickly on imperfections in the surface - so the build up will start around joints between the aluminium sheet, flat rivet heards etc.
Because composite wings have smooth surfaces, they can be designed to take advantage of the effects of laminar flow. Rivetted metal wings generally don't, so have a thicker chord to compensate.
This NASA publication concludes that
"In the laminar region of an airfoil, roughness [due to heavy rain, ice, and frost accretion] interferes with smooth flow and tends to encourage transition from laminar to turbulent flow upstream of its normal point of occurrence. In the airfoil's turbulent region, roughness considerably worsens the turbulent friction coefficient, thereby increasing the drag coefficient. Consequent dramatic decreases of maximum lift coefficient at high angles of attack lead to premature stall. Such decreases in stall angle destroy the safety margin of an aircraft approaching stall."
Even an accumulation of dead flies can slow down an SR22 or a DA40, because of the need to increase the angle of attack to compensate for the loss of lift.