Answer:
1) Big Country should specialize in the production of good X, while Small Nation should specialize in the production of good Y. Big Country can produce 1.33 units of good X for every unit of good Y that it produces, while Small Nation can produce 1 unit of X for every 1 unit of Y it produces. Therefore Big Country has a comparative advantage in the production of good X.
2) Small Nation should be willing to trade 1 unit of good X for 1 unit of good Y, while Big Country should be willing to trade 4/3 of a unit of good X for 1 unit of good Y.
3) If the production costs increase proportionality for both goods, then the country's should keep specializing in their respective productions. If the production costs of good X increases, Big Country may have to change and specialize in producing good Y. Small Nation should produce the good with the lowest costs.