Because it was reported to me by a friend w/ minis that an animal professional was advising people in an internet chat room that a HALF dose of standard vaccination was OK for minis(this was some years ago)--my vet and I did a test. We boostered one of my miniatures with a half dose of a multidose vaccine(E,W,V, tetanus, rhino, flu--the one I have routinely given for many years), waited the required time(usually a couple of weeks for 'good' immune levels to build/rebuild), then he took blood and had it tested for titers. The horse did NOT have adequate levels of immunity.
That was enough to convince me to always give full doses. As was pointed out--my 4 lb. Chihuahua gets exactly the same dose as my 115 lb. Ridgeback, and has never suffered any ill effects...there can ALWAYS be the rare exception, but overall, it seems to me proper practice to use full doses. If they were fomulated for use by WEIGHT,as are antibiotics, etc., it would be different, but, most are not!
(Of COURSE, the most important thing is to read the official instructions for use!)
JMO, and experience,