Subnotebooks are usually an inch or so smaller in both width and depth, and weigh less than five pounds. A third type of portable machine, called a luggable, can weigh up to 35 pounds and measure significantly larger than a notebook. Keep in mind that these numbers don't include the weight of external adapters and peripherals, which can significantly add to the base weight of the portable computer.
Weight tends to be an indicator of functionality.
Subnotebooks often achieve their low weight and small form factor by giving up the floppy drive, using a small display screen and shrinking the keyboard. But depending on your application, a light machine with a few basics may be precisely what you need. A CD-ROM drive will add at least one pound to the weight of a portable computer. An external floppy drive will contribute another pound or so. When you're doing your research, make sure you ask if the weight quoted is with or without the battery.