[[WMG: The Grosse Family is one of Doctor Schabbs's projects?]]
Note that the information that follows assumes that you're playing the PC/DOS version of Wolfenstein 3d, or similar.
Let's see, Hans Grosse is much larger than any other Nazi soldier, but this is also true of all the bosses in the game, with the exception of Hitler himself -- height-wise -- when Hitler's mechsuit is defeated. (Did Hitler gain a some weight?)
What partly inspired the idea for this WMG was the fact that in Episode 2 -- the episode where one fights Dr. Schabbs himself -- the pen-ultimate level has a grid-like maze with the doors to the next areas being blocks of walls that are "pushed". A few of the pathways lead to encounters with Hans Grosse, which might lead one to believe that the Doctor has been keeping some spare Grosses locked away for whatever reasons. The secret level of Episode 6 even has encounters with Hans as one of it's selling points, in addition to also reintroducing Dr. Schabbs's mutant soldiers.
Whether Doctor Schabbs had a hand in the family bloodline is still anybody's guess. Also, the maze seems more like BreakingTheFourthWall of the storyline as Apogee had a special contest planned to see who could find the "correct" path through the maze. In the end, this is just a fun wild guess.
* The official hint book really messes this theory, as interesting as it is. The Hans clones that appear in the final episode's secret level are stated in the hint book to be Hans and Gretal's identical twin brothers. So while Schabbs may or may not have been involved with turning them into super soldiers, the blame on there being so many of them falls on Mr. and Mrs. Grosse.
[[WMG: The series takes place in an alternate universe version of Nazi Germany, Spear of Destiny takes place in a different alternate universe, and at the end of that game everything reverts to normal.]]
The Nocturnal Missions begin in a world where the Nazis have a viable chemical weapon program, an undead soldier project in the works, and every military and technological advantage imaginable. BJ Blazcowicz halts the chemical weapons plot, demolishes the undead soldiers, and finally kills Hitler himself. However, a fanatical SS officer has found the Spear of Destiny (which has thus far shown no special powers) in Hitler's private quarters. He invokes the Spear's power and wishes for a world where the Nazis rule all. Unfortunately, the Spear responds by reversing time by several years, bringing all the Nazi enemies and plots toegether, and making the Spear the sole reason for Hitler's success. Blazcowicz learns of the Spear and, after fighting through hordes of Nazis, recovers it, thus ensuring an Allied victory in Europe. The rest is exactly as you've read about it in the history books...D-Day, Battle of Stalingrad, fall of Berlin, Hitler committing suicide in his bunker, etc.
[[WMG: Where does B.J. put all of his weapons in between stages?]]
The stage change elevator has a capacity of only 215 pounds, and since B.J. weights 210 pounds, he can only carry on a 2 pound lugger and 16 bullets.