ANZ bank, New Zealand's largest KiwiSaver provider, is now managing more than $10 billion in the retirement savings scheme.
Morningstar figures show the bank, which reports its half year result tomorrow, had 25.8 per cent of the $38.8 billion market as of March 31.
That means for every four dollars invested in KiwiSaver, ANZ handles $1.
Chris Douglas, director of manager research ratings at Morningstar, said he did not have any concerns about the bank's capacity to manage $10 billion.
But he said he believed the bank could be doing more to pass on savings made through greater scale to its customers by cutting its fees.
"Quite frankly I'd like to see them cheaper. They should be using the scale to bring that back to their clients."
Douglas said providers always had the challenge of trying to balance shareholder demands with the desire by investors to get the most for their money.
"We know that even small changes lead to significant gains for investors."
Douglas said it was not just an issue for the ANZ but across the industry there had been little change in fee levels in recent years.
"Aside from when you have got a default provider coming into play ... it is hard to recall a KiwiSaver provider who has lowered their fees in recent years."
Morningstar's figures show the six largest KiwiSaver providers have 85 per cent of the market.
ANZ is the largest followed by the ASB, Westpac, AMP, Fisher Funds and Kiwi Wealth.