Political Science Question
The response to this is usually to cite the reason that the Senate was originally structured to represent states equally: If big states had more representatives, they could outvote the small states. Small states need equal power in order to keep their interests from being trampled.
My question is: what evidence is there that big states ganging up on small states is a real problem in the 21st century US? The protection-of-small-states rebuttal assumes that interests and solidarity naturally fall along state lines. This may have been true in 1787, when the 13 states had been separate colonies for over a century and then quasi-independent for another decade under the weak Articles of Confederation. But I find it harder to imagine this happening today.
Imagine, for simplicity's sake, that the whole US consists of just Pennsylvania and Delaware, and that laws are made only by the House of Representatives, where representation is proportional to population. The 19 PA representatives could stick together in order to screw over the people of Delaware, who have only one representative to defend them. But that DE representative could just as easily join up with 18 of her PA colleagues to screw over the remaining PA representative -- after all, the people of the southeastern PA districts would generally have a lot more in common with the people of Delaware than with the people of far northwestern PA.
In the real US, no one state is big enough to trample the others. So you'd need a coalition of big states to gang up on an equal or greater number of small states. But what interests would unite (say) the representatives of New York, California, Florida, and Texas on the one hand, and Rhode Island, Delaware, Alaska, and North Dakota on the other?