tpa fees charged by ee count rather than asset size?

[TPApril]

When standard tpa admin fees are charged directly to participant accounts, I have seen them charged based on account balacne. A long timer participant gets charged much more on a dollar basis than a new participant. So is there such a thing as dividing the fee by number of participants and withdrawing a flat amount from each? I recognize that doing so, however, would charge a much higher %age to newer participants with much lower balances. All that being said, for a plan that insists on passing through such eligible expenses, is there a solution that seems more equitable, particularly in the case that tpa fees are charged to begin with based on participant size, rather than asset size.