NOT EVERYBODY is joining the Reagan crusade for smaller government. The architect of the Capitol, George M. White -- with the help of many assistants, advisers and consultants, and an appropriation of $450,000 -- has triumphantly produced a vastly expansionary master plan for the future Capitol Hill. The House of Representatives, it suggests, will need half a dozen more office buildings. The Senate, now completing the very large Hart Building, will be able to get along with only two more, as long as they are also very large.

What do you suppose Congress intends to do with all that space? Take in boarders to help pay its heating bills? There is the more ominous possibility that Congress might fill them up with more committees and subcommittees, their staffs and their files. All those people would be needed to produce more laws. All these laws would then require more people to oversee and improve them. The buildings would fill up quickly.

The territorial ambitions of Congress have never been modest, but this plan sets a new mark. It tactfully suggests that the Supreme Court might be happier in some other neighborhood. It offers the helpful proposal that the front yards of the houses on Second Street might be put under the legal jurisdiction of the architect of the Capitol, to ensure neatly trimmed grass.

Among its other outrages, this plan proposes to dig up the East Plaza for a gigantic parking garage. Capitol Hill has the best access to Metro of any employment center in the city -- for reasons not unrelated to the sources of Metro's funds. To spend large additional amounts of public money for underground parking for congressional staff people, to enable them to avoid using Metro, would be grotesque.

Congress, as it receives this excessively grand design, might usefully ask itself whether it really ought to double its staff, its support structure and its space over the coming decades. This master plan will only encourage all of the worst instincts of Congress. It will only incite the institution to aspire to a scale at which it would rival the Agriculture Department in size, if not in dignity.