Two months after taking office, President Carter is presiding over a White House staff that is larger than that od his predecessor and has all but abandoned his promise to make a deep cut in the size of the White House staff.

Richard Harden, special assistant to the President for Budget and organization, told a breakfast meeting of reporters yesterday that the Carter White House now has 490 full-time employees - just under the 510 full-time White House employees at the end of President Ford's term.

But on top of the 490, Harden acknowledged, the new administration has hired about 175 temporary "detailees" from other government agencies, swelling the size of the Carter White House so that it is now 30 per cent larger than the Ford White House.

Harden blamed this largely on an extraordinary volume of mail, the inclusion of energy experts on the White House staff before creation of an Energy Department and the need to beef up the White House personnel office while the new administration is taking shape.

But even when problems are solved and most or all of the temporary detailess return to their agencies, Harden conceded, the White House is now aiming at a 10 to 12 per cent cut in staff, not the 30 per cent cut the President said he would impose.

Carter made efficiency in government a hallmark of his campaign and the White HOuse staff a particular target of his jibes. He charged that the White House had become bloated under the Nixon administration and had remained so under Ford. He promised to make drastic reductions.

When Carter took office, he ordered a 30 per cent, across-the-board reduction in the size of the White House staff from what it had been under Ford. Harden said this was a completely arbitrary figure, arrived at one day in the study of the then President-elect's home in Plains, Ga., during the transition.

"Thirty per cent sounds like a good figure," he quoted Carter as saying at the time.

But since then, Harden said, the realities of running the White House have impeded the 30 per cent goal. About half the White House staff is made up of career employees who provide logistical support and other services to the First Family. There will be few, if any, cuts among these employees, Harden said.

The remaining White Houuse employees are the professional staff, filled with political appointees. Harden said the goals of the President is to reduce this from abut 275 people under Ford to about 220, providing a 10 to 12 per cent overall reduction.

Last week, the White House announced that top presidential assistants would receive pay rises but that the increases would be less than the amounts authorized. Asked if any factors "other than the obvious political reason" of projecting an image of self-sacrifice were involved in that decision, Harden replied, "No."

The pay raise decision was made by Carter's senior staff aides, who provided themselves and other top assistants with raises of up to $1,500 a year below the authorized amount while holding the raises for more junior assistants to $6,000 a year below the authorized amounts. Harden defended this practice, saying the senior aides have more responsibility.