Quirke, who recently made her on-screen Emmerdale exit, lost eight and a half stone after dieting during her time on the ITV1 soap.
Speaking on Daybreak, Robson said that between herself and Quirke, they had lost 12 stone in weight: "So quite a lot - I didn't realise that until this morning. Pauline's lost eight and a half, and I've lost three and a half."
She continued: "I saw Pauline looking so fantastic - I mean the last time I saw her she was probably the biggest I'd ever seen her, she was over 19 stone. And then she went off to do Emmerdale and then when I saw some pictures in the paper, I thought, 'My God, she looks fantastic'.
"And then I went onto This Morning and surprised her, and I just couldn't recognise her. Her whole frame - I mean you think shoulders, they're bones, they're not going to get smaller - but they have. I mean she's got this tiny little frame and I thought, 'Right, I've got to do something about it'."
The comedy duo will reunite with former co-star Lesley Joseph for a Birds of a Feather stage tour, which kicks off next month.
Discussing her friendship with Joseph, she added: "She keeps texting me 'skinny', and I keep texting her back 'fatty'! But she's not, she's always been tiny."
> 'Birds of a Feather' cast reunited on 'This Morning': Picture
> 'Emmerdale' Pauline Quirke: 'I'd love to return'