These numbers represent the size of the storage media to be used for the installation, commonly a Compact Flash (CF) card.
These are disk images, and can be written out directly to the media with a utility; There is no installation routine.
The images are identical in functionality and features, only the storage medium size is different.
A smaller image may be used on a larger card, for example a 2GB NanoBSD image on a 4GB card, in cases where the CF size is slightly smaller than the appropriate image and it will not fit. Also some cards have wear leveling will will remap unused portions of the card as they die off, allowing a smaller image to work for a longer time.