There is going to be less room at the top -- as well as in the middle and on the bottom -- in government over the next couple of years thanks to the crash diet President Reagan is force-feeding the nondefense side of the bureaucracy.

Big job and dollar cuts that could trim as many as 150,000 positions over the next two years are coming up in most cabinet departments, independent agencies and regulatory units.

While the rank-and-file cuts are in the future, most government agencies shortly will be advised that they will have fewer executive slots -- for both career personnel and political appointees -- to fill than they had anticipated.

The Office of Personnel Management, effective Oct. 1, has lowered the ceiling on the number of personnel in the Senior Executive Service. The elite $50,000-per-year SES corps had, until Oct. 1, an authorized strength (as they say at the Pentagon) of 8,599. OPM and the Office of Management and Budget have lowered the lid on SES membership to 8,272.

SES as of the end of June had 6,568 members, and the total has grown to about 6,700 today. Many of the top jobs are still vacant -- most in highly technical fields -- but not as many as before the Oct. 1 SES cutback.