As I have been made to understand it, it works like this.
The printer does not scan your 50 page job and make one pdf and then decide that it will need to divide it in 2 to fit the size limit. What it does, is guestimate the size while it is still uncompressed (Tiff, not pdf) and then splits it up and compresses to pdf. The result of which is what you see on yours.
Now with that possibly explanation out of the way, setting the split to 2 would be more likely to get good results for anything in the 100 page range or lower.