The following contains spoilers for South Park: The Streaming Wars, now streaming on Paramount+.
Eric Cartman is the most controversial figure in South Park lore. He's tortured the other boys with his xenophobia, greed and selfish attitude, with the easily-influenced Butters and Tolkien Black bearing the brunt of the drama. Cartman seemed to be somewhat maturing in Season 25 as the show shifted to the antics of Randy Marsh and Tegridy Farms... but that didn't last.
Cartman had been doing some soul-searching, especially after landing his mom Liane in poverty. However, his life was altered drastically in the Paramount+ special South Park: The Streaming Wars, which once more brought back his nasty side after Randy kickstarted a drought. In the process, Cartman underwent an outlandish and life-altering change that will surely have consequences when the show returns for Season 26.
In the hour-long special, Randy reignited his rivalry with Tolkien's dad Steve and Credigree Farms to sell water to Denver. The boys built boats to show the townsfolk where the supply came from, so they'd subscribe to the companies, but Cartman got locked into another battle with Liane. He saw a Cussler, move in nearby and begin building a mansion, which left him in awe from the hot dog-shaped shack he and his mother had been forced to move into.
Cartman realized Cussler was entering the 'streaming' game too and saw an opportunity out of poverty if Cussler took notice of Liane. He tried to convince his mom to get breast implants, like the superficial Cartman of old. He felt as a prospective trophy wife, she'd bring wealth to the family. His classmates helped out by raising the money needed for the surgery, believing that Liane was suffering from breast cancer instead.
Liane wanted no part of this plan, though. She was adamant that Cartman was being a brat, leading to a game of chicken between the two. He made it clear if she didn't have the surgery, he would, thinking that would make her give in. Well, she didn't, and Cartman ended up with breasts in an arc that felt like karma for all the crap he's given his mom over the years. He couldn't believe she hung him out to dry like that, while South Park: The Streaming Wars showed the kids at school making fun of the stubborn Cartman.
Cartman may end up getting bullied incessantly the same way he made fun of folks for their physical appearance over the years. His situation could even lead to more gender issues with him and Wendy, since he once tried to identify as a girl to gain what he thought were societal benefits. Cartman obviously doesn't have the cash to reverse the implant process, so he might have made his biggest mistake ever. It's also hard to see how this won't irrevocably change his relationship with Liane, who's finally standing up to her son's churlish mentality after years of abuse.
South Park: The Streaming Wars is now streaming on Paramount+.