InsideHook has a great collection of quotes from numerous distillery founders, owners, and curators, all of whom seem to have different ideas about what the term "small batch" really means:
Here's Trey Zoeller from Jefferson's:
“To me, small batch means tasting and choosing barrels rather than dumping every single one into a blend. [...] When you dump all the barrels you are looking for consistency, whereas when you choose a smaller allotment, you are trying to achieve a specific flavor profile in one of two ways: either by choosing a group of specific barrels with similarly unique flavor profiles, or by culling a variety of disparate-tasting barrels and then blending them together to achieve the desired result.”
Check out the other definitions in the full article linked above and keep them in mind the next time you find yourself wondering what kind of whiskey to buy.