There you go:
if this is accurate the R* have been lying all along?
It's as accurate as it gets and no they haven't been lying. RDR+SA is around the same size as V's landmass and the underwater areas (dark blue parts I assume) are roughly the size of IV and probably more so it adds up.
Also the city itself is as big as IV including water. Houses on Vinewood Hills and the small area in Northeast LS should also be counted as part of the city but people overlook them so they assume the city is smaller than it is.