And furthermore, any amendment which leaves any state with less representation than any other state has to be approved by that state. For instance, an amendment adding a Senate seat for the five largest state would have to be approved by all 45 other states.whether passed by Congress or a convention, there still has to be ratification by three-fourths of the states, and some of those three-fourths will definitely be among the small states that get disproportionate power through the Senate.
An amendment transferring powers from the Senate to the House or to some other body would not change the equal representation in the Senate, but would break equal authority in the affected powers - which arguably has the same effect.