Each district is supposed to be roughly the same number of residents - including nonvoters. So the urban districts are smaller areas than the rural districts because the population density is greater.
However the Repukes have learned that urban areas tend to elect more minorities as representatives. They don't like that - so they use gerrymandering to break up the urban areas and combine them with suburban/rural areas that have more white voters. I'm sure you get the drift.
Gerrymandering is the perfect example of incumbents (of either party) putting their thumb on the scale to make their own reelection easier. Since the 1950s it has been mostly Repukes doing it, but before that some Dems were doing it too.
It's immoral and undemocratic, but it's also harder for the minority party to oppose the gerrymandered maps. The longer it takes to get them struck down, the bigger the advantage for the majority party in the next election.
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