18.4. How much does OCR increase file size?

OCR adds between 1 and 5% to the total size of the source file if the source file is scanned in black and white.  For grayscale or color images, the increase in size is less than 1%.

If that's not making sense to you then here's an explanation and a metaphor:

Grayscale and color images are larger (in bits) than black and white images. This means that a 5-page document scanned to PDF in color/grayscale has more bits than the same 5-page document scanned to PDF in black and white. Symphony OCR, however, applies the same layer of text to both documents and that text would increase the same # of bits on each of the two scans. So, the percentage of size Symphony OCR adds actually goes DOWN the higher quality the scan gets.

Here's the metaphor: picture those scans as a sink of water (black/white scan) versus a tub of water (Color/GS). Now add a rubber duck (SOCR text) to each. The space the duck takes up in each body of water has a different percentage in relation to that body of water. The duck's percentage of space added in the tub is LESS than it is in the sink.