If you are referencing Table 5.8 from AWS D1.1, the welds sizes listed are the smallest sizes permitted based on metallurgical considerations, not based on design loads.

If there is a question with regards to the size weld required, the question should be directed to those responsible for the original design.

It is dangerous to make assumptions with regards to the design loads and the weld sizes necessary to transfer those loads from one member to another with a careful analysis of the service conditions, i.e., whether the welds are subject to cyclic or static loads, the magnitude of the loads, and the basis of design used by the original designers. Another question that must be answered in addition to those mentioned is the allowable deflection that is permitted. In a simple beam design the deflection permitted may be 1/360 of the span, whereas in machine design where rigidity is a concern the deflection may be more limited, i.e., 1/1000 of the span. The original designer would be in the best position to make those determinations.

Best regards - Al