Allardyce says Sunderland’s ex-Arsenal keeper Vito Mannone was right to quit the Gunners in 2013.
The Italian, 28, won the struggle for the Black Cats’ No 1 spot with Costel Pantilimon, who Allardyce sold to Watford in January.
And Allardyce reckons the return of England Under-21 keeper Jordan Pickford, 22, from a loan spell with Preston, has pressured Mannone into producing the goods.
Allardyce said: “Maturity is a great thing for a keeper. I think the size of the club and Vito’s age when he first went to Arsenal was difficult for him.
“When that’s the case, you’re better forging a career elsewhere. Dick Advocaat’s preference was Pantilimon, so I stuck with that for a while.
“But Vito got a chance and grasped it, and Pant went. Pick came back and challenged Vito. I think that’s made Vito play better.’’