From what you describe, I have seen similar pieces made that way. I would assume that a qualified engineer reviewed the design of the parts you are dealing with, at least I hope so. You will probably see an approval stamp of some sort on the drawings and if that is the case, I would do exactly as asked.
It is good to ask questions though, that's how we learn. Sometimes the forces acting on a weldment are coming from a different direction than we would think they will. The welds are determined accordingly.
Something you might want to look at is the "Welding Design & Fabrication" magazine; there are articles written by Omer Blodgett that address engineering aspects of welding and I think they are accessible online. John Wright posted some links to the magazine some time ago but I have not looked for that since. At any rate, those articles won't tell you enough to become a welding engineer, but they do give excellent insight as to why welds are sized and located the way they are.