Small bulkheads such as 3/4 inch and 1 inch need to use PVC pipe that is wider than the bulkhead and have an adapter to reduce down as the last fitting. Specifically the 3/4 inch bulkhead uses 1 inch PVC and the 1 inch bulkhead uses 1.25 inch PVC.
The reason for this is not clear, I can only say from my own personal experiences in developing the standpipe and trial and errors of others have validated that its required. Simply stated, using a 1 inch PVC standpipe with a 1 inch bulkhead typically yields poor results. Low flow tanks may work, but most people who tried it have reported that they needed to scrap it and rebuild to my specifications.
Larger bulkheads such as 1.5 inch and 2 inch do not have this requirement. Again the reason is not clear. An educated guess would be the reduced water friction with larger volume PVC pipes.