.............. When you oversize one part, you are in effect making all the other parts weaker, since the total weight and hence all the loads increase.............

If you have a case where in the process of oversizing one part you decide that the total airplane can now carry more loads then what you said is true. If you oversize everything then what you said is true.

I was thinking of this case in which all the other parts are adequately sized for the loads. Then just oversizing one item such as a wing joiner is usually not a big thing since the total weight does not increase that much (of course it depends on what part it is and what the increase in weight is).

Least someone misunderstands oversizing one item in itself does not make the other parts weaker, of course they can still carry the same loads they did before.