Christina for example has a few songs that were bigger than "Genie in a Bottle" like "Lady Marmalade" and "Beautiful" and that's just a few ... Does your have a song bigger than their debut single?

Beyoncé does. Halo is her biggest hit in many countries.

Poker Face and Bad Romance

Bleeding Love was bigger than AMLD

The OP's asking if your fave's début single is their biggest hit ever!

lol yeah, HG didn't even chart in US

Charli's first few singles all flopped so yes.

Poker Face & Bad Romance are bigger than Just Dance overall.

Poker Face, Bad Romance, Alejandro, Born This Way and Telephone are all bigger outside the US.

Yas . Every single one of her songs are like bigger then her debut spanish single lol
But if we are going by her english debut single "Whenever Wherever" then she has "Wakka Wakka" (10M+ WW) and "Hips Don't Lie" (12M+ WW)

Yeah. We Belong Together and One Sweet Day!

yes, some might argue her 2nd single from her 4th studio album was her biggest hit. i also think Oops...! I Did It Again was bigger than BOMT.

+ Without You.

Ellie's lead single was Under the Sheets and literally every song but Figure 8 was bigger than it.

...BOMT is definitely Britney's biggest hit.

Her debut single was so big, bigger than the singles from the majority of her peers so it would be pretty hard to top! Toxic is arguably as big/bigger!