Among the USA players, who has the lightest weight?
USA refers to nation = 'USA' ; players refers to PlayerName; lightest weight refers to MIN(weight_in_lbs);

SELECT T2.PlayerName FROM weight_info AS T1 INNER JOIN PlayerInfo AS T2 ON T1.weight_id = T2.weight WHERE T2.nation = 'USA' ORDER BY T1.weight_in_lbs ASC LIMIT 1