I stand corrected. Possibly it’s the “feel”.By "thickness", are you implying that the old Gillette razor heads were thicker or thinner than current razors?
In my view, either can be true depending on the razors.
Most of Gillette's old 3-piece razors (Old Type, Goodwill, New Improved, Tech) are very thin, while the TTO razors (Aristocrat, Fat Boy, Slim, and any Superspeed) are comparatively much thicker. and also thicker than many modern razors.
For example, a Razorock Game Changer is comparable in thickness to the Gillette 3-piece razors and both are much thinner than the TTO razors.
Of course there are also many modern razors with much thicker heads...like the Henson, Winning, and even the Timeless razors (razors I've owned).
my SLOK head is thinner then my Gillette Slim.