ROBERT H. MCKINNEY, chairman of the Federal Home Loan Bank Board, may by right when he says savings and loan institutions cannot afford to pay the same interest rate to small depositors that they pay to rich ones. But something is terribly wrong when the person with $10,000 to invest receives a rate of interest almost twice that offered to the person with $100 or, even, $1.000.

Yet, that's the way it is all over the country right now. At S&Ls in this region, the effective annual yield on passbook accounts-those used by small savers-is 5.39 percent. The same S&Ls, however, offer an annual yield of 9.57 percent to people who have enough cash lying around to buy a $10,000 six-month certificate. That means you get a little less than $27 for each thousand dollars you leave in a passbook account for six months, while you could get a little more than $47 for that same thousand dollars if you had enough money to buy a certificate.

No one defends that kinds of a spread. It exists because the rate paid to big investors follows the interest rate set by the Federal Reserve Board, while that paid to small investors is limited by federal regulation to its current level. Everyone agrees the limitation is going to have to be changed-or, better yet, eliminated-but the regulators can't agree on how to do it.

While they argue, some individuals are taking things into their own hands. Led in at least one case by a bank, groups of small savers are being formed with the purpose of pooling individual accounts before they are deposited so as to qualify for the higher rate. Other people have begun to realize that they are actually losing buying power by putting their money into passbook accounts because the rate of inflation is higher than the interest rate they are paid.

Neither the bankers nor the regulators like the idea kicking around Capitol Hill to require savings institutions to pay the same rate on all deposits and certificates. Mr. McKinney, for example, says there has to be a differential if the S&Ls are going to attract large deposits. And, no doubt, there are differences in the administrative costs of handling large deposits and passbooks.

While they are probably right about the need for a spread of some kind, there can be no justification for a spread of 80 percent. If the regulators don't end their argument fast and if they and the bankers can't find a way to give the small savers a better break, Congress should find the way for them.