did I hear my name LOL.
The easy answer to this is that shelties are smaller than collies. Shelties are NOT miniature versions of collies, though.
Here's a pic of the size difference-
http://www.collieclub.ch/img/cs/buch0103.jpg
A collie should be 24 to 26 inches at the shoulder and weigh from 60 to 75 pounds. And an in standard sheltie should be 13 and 16 inches at the shoulder. (About 20 lbs) Many shelties are oversized though due to a large influx of collie blood when the breed was still being standardized and it is still being dealt with today. I've seen some 65 lb shelties that I would've looked at and thought they were collies.
Their build is a bit different too, but it is hard to explain if you aren't in the breed. Head type is different. Collies have a more wedge shaped head and shelties are supposed to have a definite stop.
Collie-
http://collieclubofamerica.org:32143...erne_top_2.jpg
Sheltie-
http://www.shelgrande.com/Deuce_New_Ch.jpg
Collie-
http://www.tamaron.com/images/Havochead.jpg
Sheltie-
http://www.laureate.ca/Andy2yrsface11.jpg
Shelties descended from 'Toonie' dogs on the Shetland Islands used to herd sheep. Several theories as to why they were so small float around. Many believe that the animals on the Shetland Islands are miniature due to harsh climates there. There are also people that speculate the original dogs were crosses between the Scotch collie and terriers. There wasn't a real standard look there- they ranged from 'fox like dogs' to miniature collies to miniature working collies (border collie like). To try to standardize, the show collie type won out and many early breeders decided to 'help' this along by crossing to 'unregistered shelties' which were actually collies. Hence the size discrepancies and the extremely collie like look of the modern sheltie.
For fun, here's some pics of old time shelties that my Trey has descended from. (Many of these are 1/4 or half collie)
What's The Difference? 
I don't even no!!!!!!
It's-it's CRAZY!!!!! So, can you guys tell me a difference?
