The total seats / number of districts is capped and set for each state. It should be uncapped but that is besides the point

. The redistricting / gerymandering makes use of packing (putting as many X voting areas into a single district to give them less voting power by basically wasting their vote since their candidate wouldve won that distric anyways) and cracking (splitting X voting areas up into multiple districts which will be majority Y voting).

This means that with even numbers of X and Y voting people, they can be partitioned so that 9/10 districts for example are 55-45 for Y, and the remaining X voting people can be packed into the last district. The end result is Y winning 90% of the districts with only 50% of the overall support.