Crown Prince Fahd, the chief administrator of Saudi Arabia will return within a few days from a Spanish rest cure 25 to 30 pounds lighter and ready to resume his duties, Saudi sources predict.

Fahd's absence in the past six weeks, as Saudi Arabia took a harder anti-Eygpt line, generated reports that he is losing power to other members of the royal family. Minister of Information M.A. Yamani characterized the reports as "completely wrong" and said the crown prince "is still governing the country under the authority of King Khalid."

While conceding personnel and policy differences at times within high ranks of the secretive royal family, informed Saudi sources attributed Fahd's absence to physical rather than political ills-the recurrence of a back ailment complicated by exhuastion and over weight.

One source said it is "wishful thingking" on the part of Washington officials to believe that a decline in Fahd's authority accounts for the present Saudi policy on Middle East diplomacy.

Another source suggested that U.S. expectations that Fahd would arrange a more palatable Arab-Israeli policy may be used on a misreading of the crown prince's ambiguous statement, which often are aimed at pleasing his callers. The Saudi governmental budget for the coming year will reach its critical decision point in about two weeks. According to the Saudis, Fahd will be back at work in time to make the big decisions. CAPTION: Picture, CROWN PRINCE FAHD . . . loss of power denied