While the Reagan administration is trying to put the rest of the federal government on a grapefruit-and-dry-toast diet, the mess kits across the Potomac River at the Pentagon are being loaded with meat and potatoes.
As outlined in the 1983 White House budget - and consistent with the President's desire to sharply stiffen the US defense posture - the armed services are to get more of everything.
Three themes characterize the military buildup now continuing into the second year of the Reagan presidency:
* Strengthening strategic nuclear might to counter a Soviet increase that in recent years has narrowed (and in some areas wiped out) US supremacy.
* Improving mobility, readiness, and the ability to ''project power'' with conventional forces into trouble spots around the world.
* Instituting ''honest budgeting'' at the Pentagon, which means lessening those infamous cost overruns by letting Americans know that the military buildup will cost a lot now - and even more in coming years. Included here are ''management initiatives'' such as multi-year procurement and bulk buying that can be initially expensive but save money in the long run.
In his budget message, to be formally submitted to Congress today, Mr. Reagan puts it in a nutshell: ''Real defense spending will rise rapidly in the next several years, and the defense share of the federal budget will climb steadily.''
This steady climb will take the military from about one-fourth the US budget a year ago to about 37 percent in l986. As a portion of the gross national product, this is a rise from 5 percent to 7 percent.
While costs for many social programs will be held down or cut, defense spending will rise from $183 billion in 1982 to $216 in the coming year. In total spending authority (which includes money to be spent in future years and is a more-accurate measure), the Pentagon budget will rise to $258 billion, or more than 13 percent minus inflation.
This is the White House version of overall defense needs, and one that may be changed as the budget drama unfolds. While many lawmakers - including some prominent Republicans - are talking about cutting the Pentagon budget, it is unlikely that massive changes will be made.
The Congressional Budget Office (CBO) points out that even reducing the cost ''big ticket'' items like nuclear aircraft carriers and submarines, tanks, fighters, and bombers $14 billion in total spending authority only drops projected l983 Pentagon outlays by $1 billion.
''Substantially larger reductions in defense outlays in the next few years would require cuts in operating accounts,'' the CBO reported Feb. 5. ''Yet increased spending in operating accounts is generally felt to be essential for maintaining military readiness, and cuts in these accounts might therefore be less acceptable.''
With major increases for spare parts, ammunition, training, fuel, and maintenance, the Pentagon itself has no intention of scrimping on military readiness and sustainability. By budgeting for two new nuclear carriers, large cargo and tanker aircraft, 328 new Air Force and Navy-Marine tactical planes, and 776 new M-1 tanks, it wants to quickly move its influence into such regions as the Persian Gulf if necessary. To heighten the nation's power away from home, the administration also is continuing toward its goal of a 600-ship Navy by budgeting for 133 new ships by 1987 at a cost of $96 billion. This is more than twice as much as the Carter administration plan.
One of the more-controversial aspects of the defense budget no doubt will be the increased spending for chemical warfare.
On the strategic side, the administration is pushing ahead with plans for the B-1B and ''Stealth'' bombers, Trident submarines and missiles, the MX missile, cruise missiles, and improved command and control systems for these nuclear armaments.
''We think this is a minimal budget to do the job,'' says Deputy Defense Secretary Frank Carlucci. ''We don't think our budget should be measured against social programs but against the threat.''
Defense spending will be measured against the projected federal deficit, however, and the Pentagon will try to convince lawmakers that it is spending its enormous sums realistically as well as wisely.
By using a higher inflation figure based on actual Pentagon experience and enforcing procurement reforms initiated recently by Mr. Carlucci, defense officials say they intend to practice better management. Through multi-year contracting they expect to save $125 million in 1983 and more than $1 billion through 1987, for example. By budgeting $6.8 billion now for two nuclear aircraft carriers and building these new warships in tandem, they expect to save
Of course, the complicated manner in which the administration goes about buying those expensive weapons systems will doubtless come under critical scrutiny.