The size of pension funds -- and the burdens that companies assume in providing them -- have reached staggering proportions.

In 1956, Lockheed Corp. set aside $8 million to begin offering its employees a package of retirement benefits. By last year, the aircraft company's annual pension contribution has increased twentyfold, to $160 million.

Ford Motor Co.'s 1979 pension expenses totaled $811.2 million, compared with $160.5 million in 1970.

And who is footing this huge, growing bill?

"Obviously," Stan Seneker, Ford assistant treasurer said, "a pension cost is like any other cost. In order to stay in the business over the long haul, you have to price your products for it."

An estimated $136 of the price that buyers paid for each Ford vehicle last year went toward providing pension benefits for the company's 49,600 active workers and its 80,000 retirees.

The expense continues to grow as Ford, like other corporations sponsoring pension plans, is being attacked on two fronts -- one economic and the other demographic.

The most threatening economic enemy is inflation, which boosts the price of the benefits demanded by workers wanting to ensure adequate retirement incomes.

And statistically, the number of pensioners in proportion to the work-force is swelling for two reasons: a trend toward early retirement and the fact that better health care is lengthening average life expectancies.

The combination of those influences has meant that the pension fund's share of Ford's total payroll costs has grown 10 percent annually over the past decade.

Experts agree that the strict controls of the 1974 Employee Income Retirement Security Act (ERISA), make it unlikely that most private pension plans would be unable to meet their oblgations to millions of workers now expecting benefits. However, some have begun to question how long pension funds can continue in their present form, battered by forces over which they have no control.

Dallas Salisburgy, executive director of the Washington-based Employee Benefit Research Institute, said some analysts believe corporations will begin a trend toward "defined contribution" plans, rather than the now prevalent "defined benefit" plans.

Under defined contribution plans, employers would specify only how much they will set aside for pension benefits, not what those pension assets will yield for retired workers. Employees would be unable to depend on receiving a certain amount each month; instead, they would have to hope that pension investments made on their behalf today will produce enought to support them after they retire.

Other analysts have suggested that it is time for a rethinking of the whole nature of the pension system.

In a working paper, a research for the president's Commission on Pension Policy wrote:

"Historically, there has been an implicit social contract that the working generations will help support the retired and disabled generations, either privately or publicly. But the contract may have to be renegotiated . . . "

For now, such issues are matters mainly of debate and speculation. Until they are resolved, however, companies will continue to facr mounting liabilities.

Ford estimated last year that the value of all pension benefits it owes past and present employees was $8.95 billion. Thus far, it has set aside $6.63 billion to pay those costs.

Which means that $1.92 billion of its liabilities are "unfunded" -- they will have to be paid for out of future revenues. And even that enormous figure does not take into account the inevitable benefit increases that unions will win for their members in future bargaining sessions.

Ford's past experience indicates that those future hikes are likely to be considerable. Last year, the United Auto Workers won increases of as uch as 40 percent in the benefits of retired employees, adding $1.17 billion to Ford's liabilities.

Particularly susceptible to inflation are plans in which benefits are based on the salaries employees receive during their final -- and generally their best-paid -- years of employment.

One such plan is the package that Lockheed Corp. offers its white-collar workers, many of whom asre in fields where employers must pay top-dollar wages to attract the brightest technical minds. Benefits are computed according to a formula based on the top 5 years of earinings during an employee's final 10 years on the job.

Tony Ratto, director of retirement and savings plans for the firm, explained that Lockheed has been compelled by the highly competitive labor market to increase its salaries in step with inflation.

As a result, he said, its pension costs have increased correspondingly.

Lockheed's pension obligations have grown so large that Fortune magazine once described the company as "a pension plan that happens to make some missiles and aircraft on the side."

Last year, the firm reported its pension liabilities totaled $1.82 billion, an amount equal to more than 85 percent of the value of the assets it uses in its operation.

ERISA made top corporate oficials personally responsible for ensuring their pensin plans will delivery what is promised.

This new liability, combined with the astounding growth of obligations, catapulted pension plans from a minor concern to a focal point.

Chester D. Clark, a vice president in the Los Angeles office of the pension consulting firm of Towers, Perrin, Forster and Crosby, pointed out that the Internal Revenue Service refuses to allow companies to anticipate the inescapable increases they will have to make in the benefits of their union-negotiated plans. The firms, therefore, do not include those future hikes in their pension fund planning.

What the IRS is doing, Clark said, "is allowing the systematic unfunding of plans."

"Of course," he added, "that is one of those things that people allow to happen because it gives the appearance that the pension plan is not really as expensive as it is."