Discounting contributing factors like vehicle weight, tire size (primarily footprint), and power steering variances (pump pressures, etc.), the design items that contribute to steering effort are king pin inclination, caster angle, and wheel offset.
King pin inclination takes the pivot axis and tilts it in at the top (closer to the vehicle centerline) to make it intersect the tire tread centerline at the ground plane, minimizing scrub. This causes a jacking effect (literally raising the front of the vehicle) when the wheels are turned from straight ahead. Spindles that have a high angle designed in increase steering effort dramatically (case in point; the fox bodied Mustang with this angle at 17 degrees). Because this angle is fixed into the design of any given spindle, making sure the wheel/tire conbination of height and offset centers the tire's tread on this axis at the ground plane will do the most to minimize scrub and reduce steering effort provided caster angles are held to within reason. This is generally thought to be 2 to 6 degrees positive (top of the spindle to the rear) in most cases, although some straight axle designs have more designed in.
