Different design standards are going to assign different allowable unit stresses which will determine the weld size.
AISC and AWS D1.1 are intended for structural steel frames, not machinery. They consider the strength of the weld, but also have provisions to ensure the base metal isn't overloaded. Ideally, the strength of the weld and the base metal are matched. It prevents the designer doesn't overmatch the filler metal. If he does, the base metal is the weak link and the weld size has to be increased so the base metal doesn't fail.
Al