Every legal lot has a minmimum size or density to go with it. The size varies in Fallbrook as it does in other in parts of the county according to the general plan and other limitations.The state requires that all counties have a general plan.
I don’t know Fallbrook very well but it is possible some parcels have to stay very large. Perhaps because they are in ground water or septic system dependent areas,agricultural areas or flood plain areas. There definitely is a large minimum size for some parcels.Obviously many areas have been zoned for much higher density.