Steerer tube diameter
When sizing a fork to a frame, the diameter of the fork steerer or steer tube (1" or 1⅛" or 1½") must not be larger than that of the frame, and the length of the steerer tube should be greater than but approximately equal to the head tube length plus the stack height of the headset. Adapter kits are available to enable use of a 1" fork in a frame designed for a 1⅛" steer tube or a 1⅛" fork in a 1½" frame.
Manufacturers of high-end bikes, both road and mountain, have started to use tapered steerer tubes. While there are purported advantages, there are not any standards yet developed, with each manufacturer following its own conventions. This makes replacement parts difficult to come by, only available from the original manufacturer.
https://en.wikipedia.org/wiki/Bicycl..._tube_diameter
...this refers, if I'm not mistaken, to the outside diameter of the steerer, but you do run into a number of issues trying to drop a stem all the way into many threaded steerer forks, either because they are butted at the bottom, or because there is some obstruction in the lower section. If it looks like straight wall tubing, you can ream it out with the proper tool, but most people choose instead to reduce the lower stem diameter by sanding or grinding.
There's no good universal answer. If it won't go down, don't force it.