Duke Nukem 1 (1991) 271kb
Duke Nukem 2 (1993) 1.06 MB
Duke Nukem 3D (1996) 5.64 MB
So it looks as though the first two were put out by Apogee (who had some great platform games) then 3DRealms took it over for 3. Very different style games. I remember downloading the original Dukes when a 1 meg download took forever!
I played 3 as well, and Wolfenstein...but I loved the simplicity of the first two platformers. Great games to revisit.
I guess the earliest game I ever played resembling a platform was
Riddle of the Sphinx on my Atari. How far technology has come! Check out the screenshot!