The increased friction of a longer bullet with more surface area rubbing against the barrel is one factor against smaller diameter bullets when compared to larger diameter bullets of the same weight.
Another thing that works against smaller diameter bullets is the smaller surface area of the base of the bullet. The .400 bullet has a .355 bullet beat by 27% in area of the base. Pressures (Pounds per square inch) being the same between 10mm and 9x25, The 9mm projectile has a significantly smaller area for the force to act upon. Thus, less total force is exerted on the smaller diameter bullet.
The expansion factor of the gasses will be less with smaller diameter bullets causing the possibility of a different pressure curve with a more sustained peak pressure is the one thing that might HELP.
The problem I encountered with the 158 grain bullets was the need to seat them deeper due to the design of the bullet and the design of the throat of my gun. A different bullet design might lend to a longer COL and more powder capacity....
Heck, the main reason I did it was to see if 9x25 really is comparable to .357 Mag. IT IS! I have looked up 158 SWC data on the web. My effective revolver length barrel is about 2.5" (Subtract the COL from 3.8"). The velocity of 1300 FPS with a 158 grain bullet is typically quoted for 4" to 6" long barrels in 357 Magnum!
Are you doing Things Right?
Or doing the Right Things?