MKI & II Disk Brake Rear...cheap option?

I have been reading through every disk brake conversion thread I can find on the many Escort forums. Seems to me that everybody likes to use wierd and wonderful brakes from many different manufacturers.
There is obviously a very easy answer to this but..........
When upgrading the front disks on an Escort, why can the original front disks and calipers be used on the back axel? Surely original Escort fronts are no more oversized than some of the other options used (eg Pug 105 fronts)?
Anyway, just trying to save some dollars......
There is obviously a very easy answer to this but..........
When upgrading the front disks on an Escort, why can the original front disks and calipers be used on the back axel? Surely original Escort fronts are no more oversized than some of the other options used (eg Pug 105 fronts)?
Anyway, just trying to save some dollars......