Colt probably made it heavier 'cause heavier rifles hold still better and makes accurate shooting easier.
Barrel weight has nothing to do with accuracy. Each one whips the same for each shot regardless of its length, weight or stiffness. Stiffer barrels whip less but whippier ones whip just as consistant from shot to shot. The rigidity of barrel steel doesn't change as it heats up unless it's hot enough to soften the barrel so it'll burst when fired.
Nor does barrel weight have anything to with changing the bullet's point of impact as they heat up. Any barrel that changes where the bullet goes suffers from at least 1 problem. The barrel's not stress relieved correctly and stress changes bending the barrel as it heats up. If the barrel is perfectly stress relieved but still changes point of impact as it heats up, then the receiver face isn't square with the bore axis; as the metal expands from heating, the high point where the barrel mates with the receiver bears hardest on the barrel causing it to bend in that direction. If both issues exist, then the impact point changes a lot more for a given amount of barrel temperature increase.