The biggest farmers are getting bigger, small farmers are becoming more numerous and the guys in the middle are getting squeezed out, according to preliminary data released last week by the Census Bureau.

In their cold, hard statistical way, the findings reported in the 1982 census of agriculture tend to verify the concerns about farm structural change that have swept the country for years.

For example, while the census found only 16,651 fewer farms in 1982 than the 2,257,775 it reported in 1978, the statisticians identified significant changes among those farms with $1,000 or more in annual sales -- the standard used to define a farm.

Farms with 50 acres or less increased by 17 percent in that four-year period, to 637,000. And farms with 2,000 acres or more grew from 63,301 in 1978 to 64,525 in the new study.

But major changes were occurring in the opposite direction among farms between 50 and 999 acres.

Their number went from 1,553,887 in 1978 to 1,442,137 in the new census, a decline generally attributed by farm experts to operating difficulties tied to interest rates and low commodity prices.

"There has been a significant decline in this category," said George Pierce, acting director of the agricultural census. "They tend to be full-time operators, but they don't have the capital to continue farming."

These were some of the other findings:

*The average American farmer was 50.5 years old in 1982, compared with 50.3 in 1978.

But the census found 4,200 fewer farmers 25 and under in 1982, while there were 29,000 more farmers 65 or older.

High interest and land costs have slowed the entry of younger people into farming, most experts agree.

*Almost half of the farmers, just over a million of them, reported that they considered their principal occupation something other than farming, although 646,000 of them were living on the farms they were operating part time.

*There was an 8 percent increase in the number of women farmers, who in 1982 made up 5 percent of the total farmer population and were farming 35.4 million acres of land.

Pierce said the new figures expand on a trend first noted in the 1978 census.

*The number of black farmers continued a decline that began early in the century, falling 11 percent from 37,000 to 33,000 in 1982.

Hispanic farmers declined from 18,000 to 16,000, the census showed.

*The vast majority of American farms, 1.9 million, were still operated by individuals or families.

But the number of farms operated by nonfamily corporations increased from 5,818 in 1978 to 7,131 in the new census.

Other types of nonfamily ventures -- cooperatives, trusts, estates -- grew from 9,146 to 12,273.

*Texas, although third in the nation in agricultural sales, still has more farms than any other state (185,026), and between 1978 and 1982 it showed the biggest increase in small farms -- up 14,109, or 46 percent, to 44,587.

Vermont had a 59 percent gain in small farms.

In other ways, the census showed that a relative handful of large farming operations dominate the U.S. food-production system.

Only 1 percent of the farms, for instance, sold 500 or more head of cattle and calves, but they accounted for 43 percent of all sales.

Farms selling 500 or more hogs represented only 16 percent of all hog operations, but they held 70 percent of the market.

Farms with more than 20,000 hens and pullets made up just 2 percent of poultry producers, but they had 79 percent of all inventories.

Which, for the small-scale poultry producer, is probably nothing to crow about.