That first one is caused by the weight on the nosewheel, not the weight of the nosewheel. It's a proportion of the entire aircraft weight that is carried. The greater the percentage for the nose gear the heavier and more rapidly one would have to move the elevator on landing to ease the nose down or have to make only 3-point landings (or 18 points, apparently, for 747s).
The reduced size is more a result of the desired weight distribution leading to the main gear carrying more weight and seeing more wear as the primary landing contact patch - highest speed, greatest wear. Since the nose gets less abuse and less wear they reduce the size and weight to lower the overall weight of the aircraft.