Why is Kiwibank so much smaller than the "big four"?

The topic of New Zealand-owned banks is arising again, with NZ First pitching to buy back BNZ to bolster competition. 

Kiwibank is already New Zealand-owned, but only holds a little over 6% of the lending market, raising the question – how much do Kiwis actually care about backing a Kiwi bank? 

Kiwibank CEO Steve Jurkovich told Kerre Woodham that they've been in operation for much less time than some of those bigger banks —around 25 years compared to over 150— so they've had less time and capital to grow. 

Jurkovich told Woodham they could always be bigger and have more impact, but over the last few years they've been growing pretty strongly. 

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