Strength modifies your carry weight, melee damage, and accuracy for ranged weapons. Power armor doesn't require a certain strength level, it just increases your strength. If you use large weapons but don't meet the strength requirement you just won't be as accurate but it doesn't affect the weapon's damage unless it's melee. I don't know exactly how inaccurate you'll be if you don't meet the strength requirement. And Big Guns from Fallout 3 was just merged with guns, energy weapons, and explosives in New Vegas.