It is true that few are as big as Hitomi, but there have been an awful lot of D-cup or bigger gravure models coming out of Japan in recent years (far more so than previously). This is highly suspicious statistically, and the fact that this coincides with the wide availability of breast surgery only makes it more so. On top of that, the fact that they don't take their tops off only adds to the suspicion (as it may be to hide signs of surgery, such as scarring) - when a real, undisputed natural wonder comes along (such as Anna Ohura or Jun Kusanagi), they usually don't have any qualms about taking their tops off.
Of course none of this actually proves anything, but it is highly suggestive that there is something more going on here than just mother nature.