If you're among those Americans who ate hamburgers yesterday or Tuesday as a change from a heavy Christmas dinner, you may have digested the bargain of the year - and of 1979 as well.

After surveying the beef market, the American Meat Institute yesterday predicted that hamburger prices would jump another 40 cents to 50 cents a pound next year, making ground beef cost as much as round steak did last year.

Steak prices aren't expected to go up as much, proportionally, according to the industry trade group. But institute President Richard E. Lyng said they may said as much as 20 cents to 25 cents a pound.

Hamburger prices rose an average 37 cents a pound in 1978, to $1.18 a pound in October. Steak prices jumped an average 56 cents a pound this year, to a new level of $2.98 a pound for porterhouse cuts.

Experts say the reason is the age-old imbalance between supply and demand: the long-expected rebuilding of cattle herds in the wake of the 1974-75 recession hasn't materialized, leaving a good many feedlots short.

Because of the continuing shortage, cattle producers are expected to slaughter more of their fatted steers for choice meat cuts, leaving less for processed meat and ground beef. So hamburger is expected to be in short supply.

Industry leaders had counted on increased pork and poultry production to take up some of the deman, but disease and other problems have clouded that outlook some.

The Agriculture Department has predicted that pork production still will increase sharply.

Government economists have been predicting that hamburger prices would rise 10 cents to 15 cents a pound in 1979.

If hamburger prices do rise to $1.70 to $1.80 a pound, as the meat institute predicts, it would make the nation's favorite short-order food about as expensive as round steak. Round steak sold for $1.77 a pound in February.

As in similar cases where demand outstrips supply, a sharp jump in prices ultimately may prove the fastest solution - by dampening demand for hamburger meat and forcing consumers to turn to substitutes.

Lying yesterday predicted that if prices leap as much as the institute is forecasting, meat consumption in the United States will drop next year to an average 180 pounds for each American - six pounds per person less than this year.

At the same time, Lyng said poultry consumption should increase, to 60 pounds a person from 57 pounds this year. Meat production is expected to fall to 38 billion pounds in 1979, from 38.7 billion pounds in 1978.

Lyng said beef prices will begin rising next spring, and will be sharply higher by midsummer. At the same time, he said, pork prices aren't expected to drop until late in the year - even though production is apt to rise.

But he declined to predict when the p "I'm always uncertain as to when the consumer will decide that 'this is enough,'" he said. "We simply can't predict it with any degree of precision. . . "

Beef prices have been rising rapidly since early autumn, jumping sharply in November and Devember after a brief respite earlier in the year. Cattle prices soared last spring but then tapered off.

The Carter administration has predicted that overall food prices at retail would rise by 6 to 10 percent in 1979. Most private analysts are forecasting an increase near the higher end of that range.

Only a few months ago, analysts were predicting the beef price rise would begin to taper off soon, on the notion that ranchers ahd stopped liquidating their herds and were about to begin rebuilding. But the start proved false.

The fluctuations stem from the 1972-73 boom, which sent beef prices soaring and later left ranchers caught short when the market collapsed during the 1974-75 recession. Most cattle producers have yet to rebuild their herds.