I recall hearing that they use the same internals as the 1" scopes, so don't really have the advantages of a true 30mm scope.
Actually, I did a bit of a quick search, and came up with this:
http://www.opticstalk.com/forum_posts.asp?TID=1126&KW=30&PN=0&TPN=2
30mm scopes were first made in Europe years and years ago for two reasons.
1. Everything was done by hand and a large tube was needed to be able to build the scope.
2. They are on the metric system.
When we started building scopes in the U.S. technology had advanced some and being on the Standard system we built our scopes with 1" tubes.
The German scopes were and are superior because they use better glass, better coatings and better craftsmanship. Most people assume that bigger is better and that must be the reason why German 30mm scopes are brighter. It did not take long for U.S. and Asian scope makers to catch on to this and start offering 30mm scopes. It worked for a while but more and more people are learning the truth with the advent of the Internet. When Leupold first offered a 30mm tube in a hunting scope, they named it the Euro. 30. A competitor dissected one and said, B.S. that scope has 1" guts with a 30mm tube. Leupold's spin doctors quickly changed the name of the new 30mm line of scopes to LR or Long Range and released press releases stating that this new line of scopes has 1" internals with a 30mm tube to allow for more elevation adjustments.
Bottom line is 30mm scopes are 30mm because of the Metric system, they can be brighter than a 1" scope with all other things being equal (# of internal lenses and diameter of objective especially) if the maker takes advantage of the larger internal lenses in a manner that manages the light better (cutting down on distortion and stray light loss). 30mm scopes are also stronger and usually heavier. Biggest advantage is more internal adjustment travel for long range shooting.
Of course, it is all still hearsay, so YMMV.