Preslotta isn't uniform, just as post slotta isn't. The earlier Fiend Factory era stuff is more like true 25mm; wheras the 1984 date stamped stuff was 28mm and stockier. The ranges that saw the transition - hobgoblins, chaos beastmen, half orcs, lizardmen etc rank up as they are essentially the same miniatures, but don't bear much relation to the >32mm stuff in the early 90's.
If you're talking 1984-1987 pre / post slotta then head to foot on average they are within a mm in terms of scale.