Wikipedia, the most reliable source on the internet, confirms that the 100 is basically a mini portable. A quote from the PowerBook 100 series page:

The PowerBook 140 and 170 were the original PowerBook designs, while the PowerBook 100 was the result of Apple having sent the schematics of the Mac Portable to Sony, who miniaturized the components.

Some other sources also say the same thing.

(Edit to try and remove the link to the Sony Wikipedia page. I failed, so if you want to go to their page click the S. :lol: )