WHILE SOME cassette recorders are getting bigger and more elaborate, a strikingly opposite development is about to come on the market.

The new product is a cassette deck that weighs about one pound. It is small enough to slip into a large coat pocket. You listen to it via ultra-lightweight headphones which themselves weigh under 4 ounces. And it is sterophonic. It will accept the same cassette system. Prices vary from about $70 to more than $250, depending on features and capabilities.

Nearly two dozen companies have announced these new systems and while, at this writing, prototypes are more in evidence than actual products, the general attitude toward the petite performers is optimistic. Individual manufacturers indicate they expect to sell hundreds of thousands, and total sales estimates run as high as 5 million units for the coming year.

The basic format is similar in all models. The cassette fits into a normal-size compartment, and the entire device is not much larger than that compartment, with small controls for starting, fast-wind, stop and volume. The headphone cord is connected, via a mini stereo plug, into a small jack for another listener. A few units will provide a recording function; these of course weigh and cost somewhat more than the playback only versions. Some manufacturers plan to offer an "FM cassette" which converts the device for receiving stereo or mono FM broadcasts.

Few units are available for examination. One is the Sony Walkman, which can be credited with having started this trend. Then there is the Sony "Pressman" which includes the recording function and was developed originally as an on-the-spot tool for Japanese newsmen. A third unit is a Sanyo version known only as the Model M5550. All three may be run on self-contained batteries, or -- with adaptors -- from car and household electrical sources or a rechargeable battery pack.

They all sound amazingly good. I suspect that their internal circuitry has some treble boost since the units all have tone controls desgined to cut down on the highs. This seems to be an attempt to compensate for the units' lack of Dolby and/or to equalize for high-performance tapes. The Walkman and the Sanyo use a simple "high-low" tone switch; of the two, the Walkman's seems more effective. The Pressman sports a variable control, which is best.

The Walkman and the Sanyo have a control that permits the wearer of the headphones to reduce the volume of the unit in order to hear something from outside. This is supposed to be a safety feature for use during cycling, jogging, skiing and so on. But only a really loud or very close sound can be heard by the headphone wearer. The Pressman, on the other hand, which is designed for recording, does pick up external sounds quite well, but only if it is loaded with a cassette on which you can record (not a commercially prerecorded cassette). When you use this option, you erase anything that had been recorded earlier.

Mechanically, the three samples worked well, although I did encounter a few annoyances: The Walkman's eject mechanism seemed sluggish, and the Sanyo's battery contacts had to be bent upward slightly to connect properly to the batteries. Of the three models, the Sony Pressman seemed the best -- in terms of mechanical functioning and of sound.