What we need is a clear question.
If your bullet does not simply drop into the case (i.e., if the inside diameter of a .45ACP is less then the 0.451" diameter of the bullet) and if your slide has enough force to jam the case in the chamber, then you may not need to resize.
However, in most cases, if the case mouth is NOT larger than bullet diameter, then you are shooting a very low pressure load or the outside case diameter is too large or the chamber inside diameter is too small (you may have ammunition that is too tight in the chamber to allow the case mouth to open enough to release the bullet without chamber pressures going up to a dangerous level).
Of course, my own feeling is that if 99.999999999% of reloaders always resize their cases, there might just be a reason for doing it.
This falls apart, however, with cast bullets. Almost all bullet caster size their cast bullets. I haven't done that since the mid '80s and have not seen any reason to go back to that time-consuming and, often, accuracy-destroying activity.
Somewhere in the rant, I hope I addresses whatever the real question was.
PS: You only trim RIMMED straightwall cases , that head-space on the case rim, so there is a consistent case length for roll crimping. Semi-autos use a taper crimp and the cases never need trimming and they are, in fact, too short to begin with and shrink with use...