Dragon Ball: Why Android 18 Is So Weak Compared to 17

screenrant.com:

Dragon Ball Super made it clear that Android 18 is several levels below Android 17 now. In Dragon Ball Z, the two were shown to be on an equal playing field. During their first story in the anime, 17 and 18 battled hard against Super Saiyan Vegeta and Piccolo respectively, with both villains proving to be formidable opponents.

Since then, both characters have gone through some notable evolutions. At this point in the franchise, Android 17 and 18 have left their villainous roots far behind them. In Dragon Ball Super’s Tournament of Power, the brother-and-sister duo fought on Goku’s Universe 7 team and managed to work together effectively. In fact, 17 proved to be so powerful that he lasted until the end of the tournament. When Goku and Frieza sacrificed themselves to take out Jiren, Android 17 was left as the last fighter standing – and in turn, the winner of the tournament. What he was able to accomplish would never have been possible if he hadn’t made some shocking, off-screen gains. His fight with Goku proved that the character is currently Super Saiyan Blue-level.

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Meanwhile, 18 hasn’t been able to match her brother’s impressive progress. Her fights in the Tournament of Power weren’t a clear indicator of where she sits on the power scale, but her failure to beat Majora (who was defeated by Krillin) created the impression that though she is stronger than her husband, Krillin isn’t that far behind her. That means 18 is now grouped with the weaker Z-Warriors, whereas 17 is up there in the same class as Gohan and Piccolo. 18’s failure to climb as high as 17 in Dragon Ball Super can most likely be attributed to the differences in their lifestyles.