PizzaExpress, the 300-strong chain, is increasing the size of its pizzas by roughly 20%.
The company has been plagued for years by claims that its pizzas have got smaller, but has always denied this.
But after testing out the larger pizza at its branches in Ireland, the company has this week introduced the new-look pizzas in the UK. Prices will remain fixed, however.
A spokesman for PizzaExpress declined to reveal how much the move would cost.
"We've had it trialled in Ireland for several months," he said. "Whether it costs the company or not is irrelevant. Keeping our customers happy is what we want to move towards."
The company also denied any connection with the recent abortive £250m bid for PizzaExpress by Hugh Osmond, one of the key players behind the group's flotation on the stock exchange in 1993.
If Osmond had won control, one of the first things he pledged to do was increase the size of the pizzas.
"It's not a knee-jerk reaction," said the PizzaExpress spokesman. "It's something we've been considering for some time. With a group like ours, it's not something you just do. It's something that has to be considered at some length."
As well as boosting the size of its pizzas, the company is introducing one new variety with a Cajun chicken topping, called Pollo ad astra. It will also add a chicken Caesar salad to the menu.