from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Small enough to fit into a vest pocket: a vest-pocket book.
- adj. Very small; diminutive: a vest-pocket park.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Small enough to be carried in a vest pocket or other small container.
- adj. By extension, anything smaller than would ordinarily be expected.
From vest + pocket, referring to the pockets of a suit vest, which are only sized to hold watches, handkerchiefs, wallets, and similarly small items. (Wiktionary)