Despite President Carter's pledges to reduce the size of the White House bureaucracy, the administration is backing legislation authorizing the president to double the size of his top personal staff.

Presidential aides said the move was designed to provide "flexibility" for the future and does not portend an immediate increase in the White House staff, which they said has been trimmed by nearly 30 percent since Carter took office 13 months ago.

"Authorizations are always higher than what really happens," said pressaide Patricia Bario.

The legislation, which was approved by the House Post Office and Civil Service Committee last month and is scheduled for a House vote on Tuesday, generally authorizes staffing levels and procedures that are already in effect, according to Charles H. Knull, staff counsel for the subcommitted that drafted the bill.

The major exception is authority for hiring senior prsidential aides, the executive-level and supergrade staffers who generally receive annual salaries ranging from $42,500 to $57,500, Knull said.

At the request of the White House, the proposed legislation would increase the number of these slots from 55 to 100, he said.

Negotiations between the House committee between the House committee and the White House resulted in some provisions for limited congressional review of presidential staff spending.

"They said they wanted flexibility," said Knull. Hugh Carter Jr., special assistant to the Presient for administration, was quoted by the National Journal as saying, "This provides permanent statutory authority for the presient's staff. We wanted to have plenty of flexibility for the future, in the event of an emergency like World War IIIor something, and needed to suddently increase the staff. But except for one or two people, we have no plans to raise the staff level."

Counting all proposed authorization increases for top aides, including other units of the Executive Office of the President as well as the White House personal staff, the cost increase would be $8,6 million for the first year - if all the slots were filled.

This would give a total cost of $29.5 million for 1979. But, according to Bario, the White House is requesting $16.9 million for the year, considerably below the proposed authorization ceiling. This, she said, is in line with Carter's 1977 reorganization plan that resulted in reducing the total White House staff from 485 to 351.

The legislation, which updates a 39-year-old, drastically out-of-date authorization bill for White House staffing, would also give the president virtually open-ended authority to hire consultants, borrow "detailers" from other agencies and appoint staffers at less than a GS16 yearly salary of $42,423.

These and other authorizations, including staffing for the wives of the president and vice president, conform to existing practice although they go beyond the last White House authorization bill, which was passed in 1969, according to Knull.

Normally appropriations are kept in line with authorizations, and it was not until the Nixon administration, when the White House staff mushroomed, that staffing became an issue. Two attempts to update the authorization failed, but this one is expected to have more success in light of support from the Democratic administration and at least some congressional Republicans.