The senior administration officials who challenged the Pentagon's ambitious expansion plans last year in a hard-fought internal budget battle have backed down this year in the face of President Reagan's strong determination to continue the defense buildup in his 1984 budget.

The result is that Reagan has decided to send a budget to Congress next month that advances his goal of a larger military. He is undeterred by repeated bipartisan warnings from Capitol Hill that the Pentagon should not be exempt from budget-cutting.

Reagan has said repeatedly that his military buildup is more important than the need to control record-breaking deficits.

"He has made his position very clear and it is the same position in private as it is in public," said a senior administration official, who asked not to be identified.

The official said there is "no inclination" among Reagan's top staff to attempt to change the president's mind this year.

Some Reagan aides say they believe that the Republican-controlled Senate will take the lead in trimming the costly defense buildup. Even if this happens, they emphasize, there is no certainty that Reagan will accept anything more than cosmetic cuts in the Pentagon budget.

White House officials foresee no repetition of last year's in-house fight over defense outlays. In that confrontation, Defense Secretary Caspar W. Weinberger convinced the president not to scale back his military buildup, as was advocated by Office of Management and Budget Director David A. Stockman and White House chief of staff James A. Baker III, among others.

Reagan will get a budget review briefing for the Pentagon shortly. He has not signed off on final defense spending decisions, but given the inclination of key administration officials not to make a big issue of it this year, the general course Reagan will take is all but assured, these officials said.

Still, Reagan must resolve two outstanding defense issues that could result in somewhat smaller defense outlays:

* The so-called "deflation dividend." Some administration officials believe that because the pace of inflation has slowed, fewer dollars are required to maintain the same real growth rate in Pentagon outlays. Thus, they think Reagan can keep his promise for at least 7 percent growth in the military budget with less money than was envisioned last year.

The Pentagon is still resisting the idea. But informed administration officials predict that such an adjustment for declining inflation is likely to be written into the budget.

* Whether Reagan should remain within the broad categories for defense in fiscal years 1984 and 1985 that were set down in last summer's budget resolution. The president declared in July that he would abide by the overall budget ceilings, but not by the specific defense levels, as projected for 1984 and 1985 in the budget resolution.

The Pentagon is standing firm with the view that Reagan should not be restrained to specific defense levels the resolution prescribed, but other officials think the president should not break them.

In both these issues, however, the amount of potential savings is modest for fiscal 1984. If the president decided both to adopt the inflation adjustment and to stay within the budget resolution, he would realize about $10 billion in 1984 savings from previously projected levels, officials said. This would become $18 billion by 1986 and $25 billion by 1988, they added.

The budget Reagan sent to Congress last winter envisioned Pentagon outlays growing to $247 billion in 1984. The administration's mid-year budget review in July estimated $253 billion for 1984 out of a total $812 billion in federal outlays.