This depends on i) whether the source images are 24 bit with unlimited colors or 2, 4, or 8 bit with limited colors ii) the PixelStore internal format used (8 or 24 bit JPEG or 1, 4, 8, or 24 bit PNG) and iii) the PixelStore compression level setting.
For example, export of 24 bit TIFF aerial photos to a PixelStore 24 bit JPEG layer with the default JPEG compression setting would likely result in a PixelStore layer that is much smaller than the source data. (The size reduction, along with some quality degradation, is due to the JPEG compression.)
In one customer example, 40+ GB of uncompressed, gray scale, TIFF imagery (composed of approximately 1,200 individual TIFF files) exported to a 5 GB PixelStore 8-bit JPEG mosaic. (This example involved a reasonable JPEG compression setting and no pixel resampling, i.e., the exported PixelStore layer contained the same number of pixels as the source data.)