The PowerBook 100 was a big step forward from the Mac Portable. First and foremost, at 5.1 pounds it was only one-third the weight. Secondly, it was several thousand dollars less expensive. The weight reduction was accomplished by using lighter plastics, eliminating the floppy drive, and using a smaller (2 hour) lead-acid battery. Cost savings were achieved by eliminating the floppy drive (a $200 accessory), using a smaller hard drive (20MB), building around a 9" (640 x 400 1-bit) passive-matrix screen, and having Sony manufacture it. (Sony designed the 100 by starting with the Mac Portable and reducing the size and weight of components as much as possible.)
This was the first PowerBook with SCSI Disk Mode, which allowed desktop users with a special cable to connect the PowerBook and use it as an external drive. (The 140 and 170, introduced simultaneously with the 100, lacked this feature.)
Until Apple discontinued the PB 100 for under $1,000, it was a very poor seller.