BY SIGNING off on Defense Secretary Weinberger's token reductions in the Pentagon's budget buildup, President Reagan has dampened congressional hopes for forceful action against the budget deficit. The president will certainly continue to press for further cuts in the domestic budget. But the impression is running strong that, apart from his personal commitment to reducing certain social programs, the president is not especially concerned about the yawning gap between what his government spends and what it takes in.

The argument within the administration over the military budget has not been without educational benefits. It has reminded everyone just how big the Pentagon's budget is -- and how rapidly it will continue to grow if the president has his way. In the fiscal year that started in October, Defense has authority to spend $285 billion. Because of lags in letting contracts, moving weapons into production and so on, much of that money won't actually be spent. But using authority from previous years, the Pentagon will spend about $254 billion.

In the next fiscal year, Secretary Weinberger had wanted to increase his department's spending by more than $32 billion. Now he has agreed to keep spending at $278 billion -- an increase of roughly $24 billion over this year. This increase would cancel out almost all the savings that OMB hopes to realize from canceling and cutting domestic programs. Three years hence, Mr. Weinberger still hopes to have an annual budget of more than $400 billion, of which about $350 billion actually would be spent.

Another reminder from the recent exercise is how much padding the Defense Department can put in its budget. Of the $8.7 billion in 1986 savings now being offered, more than $1 billion comes from counting a civilian pay cut that OMB had already counted in its savings. About $2.6 billion comes from giving the military an extra pay increase in 1985 (apparently easily absorbed in the excess fat in this year's budget) and then skipping a scheduled raise in 1986. Another billion comes from acknowledging, for the third year in a row, that the Pentagon has inflated cost estimates for fuel and other purchases. A final $2.5 billion comes from funds still unallocated to specific purposes.

This sort of number-juggling is not likely to increase Congress' confidence in the Pentagon's budget estimates or in the president's commitment to dealing with the deficit.