House seats are apportioned according to each state's population. So a populous state like CA, TX, FL, or NY will have more House seats. The Senate is two votes per state, so that a small handful of populous states cannot use Congress to exploit less populous states.
We all learned this in junior high or high school, even those among the DU-folk who stayed awake and un-stoned in school. So, what's the real beef? The Ds' base constituencies are concentrated in large cities. And how many DU-folk dwell in or near large cities? The DU folk want to be able to do exactly what the Senate was set up to prevent. The DU folk want to be able to run the US based on just the votes of a few large cities, such as NYC, Chicago, LA, Houston, Miami, Philadelphia, Seattle-Tacoma, and SF.