The singer - who was a runner-up in this year's Strictly Come Dancing - is renowned for her womanly physique and told Cosmopolitan magazine that Nicola is her biggest fan.
Kimberley said of her shapely derriere: "It's got a life of its own! Something about my bum was trending on Twitter recently and I was like, 'Oh, my God, you lot are crazy!'
"Nicola loves to give my bum a squeeze too. It's very toned and a completely different shape so she's like, 'It's a sponge!'"
Kimberley also confessed the band has "fallen back into the same patterns" since they reunited late last year.
She added: "Well, Sarah's not really a party animal any more so that's changed, but I still fulfil the same role. Like, on our first performance everyone was getting really nervous and I was thinking, 'Right, OK, let's try to calm this situation down.'
"Cheryl's quite motherly too, while Nicola and Nadine are the young ones. We're still protective of them even though they're women now!"