The reason is not because we have bigger horses. Jumpers have some really big horses to, and they are still using long girths - explain that.
Big black furry horses are also carriage horses - not dressage horses (but that's a whole different can of worms I'm not getting into). They just started doing dressage in like the last 10 years - modern dressage saddles have had long billets far before that.
It is to reduce bulk under the leg, the same idea as the monoflap designed saddles, etc. In dressage we use such minute aids that the less saddle (and girth) there is between horse and rider, the easier it becomes. I find having the long billets also lets your leg lie straighter than in a short billeted saddle.