It wasn't smaller, it was similar to what is now the standard DVD case size, although a little taller. Construction was very similar to a CD jewel case though, with a separate front/book and spine/back paper parts, and the casing itself all made of clear plastic. iirc the original DVD release of Lock Stock & 2 Smoking Barrels was the same.
In the early days of the format they didn't seem to have decided on the black plastic (amaray) cases that everyone uses now – I imported quite a few R1 DVDs and lots of those were in the "snap" cases with the cardboard front and the side bit that snaps in place.
http://www.imdb.com/Sections/DVDs/Packaging/