Not all buyers of small cars today are getting their money's worth. Many compacts, especially those with automatic transmissions, don't get much better gasoline mileage than top-performing larger cars. When you crank in the effect of the big price discounts now availabe on intermediate and full-sized models, the bigger cars may be the better buy.

This unique situation springs directly from the gas shortage last May and June, which struck fear into the hearts of car buyers nationwide. Small cars amount to 60 percent of sales in June, compared with 50 percent last January. There's a waiting list for most compacts, while a record number of intermediate and full-size cars go unsold. Manufacturers have even delayed introducing the 1980 models by two weeks (to early October), to help clear 1979's white elephants off the floor.

They're also offering incentive payments to dealers who sell more than their quotas. General Motors, for example, gives dealers $100 to $500, depending on volume, for each extra Oldsmobile sold, and $200 to $700 for each Cadillac. Ford and Chrysler make similar payments.

A car dealer's gross profit normally runs in the area of $200 to $300 a car. Given the manufacturer's cash incentives, a dealer can now sell big cars for less than his own cost and still make a normal profit.

"If you can't sell a big or intermediate car today for dealer's cost," a spokesman for Pontiac in New York told me, "there's something wrong with the shop."

Compacts and subcompacts, by contrast, are in such demand that it's hard to get a nickel off the list price. Some cars sell for more than a full-sized car. Even if the compact gets better gas mileage, the difference may not be enough to offset the higher list price.

As an example, compare a full-sized Pontiac Bonneville, loaded with options, with a Phoenix, Pontiac's hot-selling version of General Motors' new X-bodied compact. The Bonneville can be had for $7,700, which is dealer cost. The Phoenix, with similar options, sells for its sticker price of $8,285. That's a $585 difference.

With an eight-cylinder engine, the Bonneville gets 17 miles a gallon in city driving, as measured by the Environmental Protection Agency. The automatic-transmission Phoenix gets 21 mpg - not very much more.

Assuming an average gasoline price of $1.10 a gallon, 10,000 miles of driving would cost $647 in a Bonneville and $523 in a Phoenix, for a small-car saving of only $124. You'd have to drive about 47,000 miles before the Phoenix would pay for itself in gasoline savings.Assuming $1.25 a gallon, the breakeven point is a little less than 47,000. For the average car owner, that's somewhare around the fourth year of ownership.

The price-mileage comparison isn't everything. Big cars may cost more to maintain and repair (although that isn't always the case). They give you less range than a compact if another gas shortage strikes. Many smaller cars have front-wheel drive, which is better in snow. Small cars are easier to park.

If you're happy with a stick-shift car getting 30 to 35 mpg, it's no contest. Subcompacts cost less than big cars and yeild good gasoline savings. But drivers considering comfortable compacts, with air conditioning and automatic transmission, may do just as well buying something larger.

The current big discounts on intermediate and full-sized 1979 cars should last until the supply is gone - an estimated 200 selling days from now, according to Patrick Bedard, editor at large of the magazine, Car and Driver. Where larger cars are popular, the supply could last no more than two months.

In making price-mileage comparisons, don't be misled by the big "highway driving" mileage figure shown in the ads. That's much better mileage than most drivers acutally get in practice. The Environmental Protection Agency now uses the "city driving" figure as its official estimate - which may be a little low, but comes closer to the mileage you'll probably get.