There are a lot of reasons that a 1 or 1.2 meter usually outperforms a BUD for KU only. The biggest reasons are surface accuracy, feed type, and alignment.
Most people will use a mesh dish because glass or spun dishes are so much harder to install because of the weight. The mesh dish probably has distortions and dings that adversely affect KU signals while not affecting C band signals as much.
A prime focus feed is needed to allow the feed to see the whole dish. Some folks try to put an LNBF there because that's what they have handy, and performance will be degraded. A prime focus feed with a proper LNB will outperform even a Corotor...
Dish alignment is 4 times more critical on KU than C, so some tweaking is required to get it spot on.
I use an 8 foot Channel Master SMC as a KU only dish, and it blows my 1.2 meter CM's and Prodelins out of the water, and I've seen a couple of signals that it edged out my 10 foot CM with Bullseye II/PLL LNB's.
I wrote an article many years ago about what to look for when searching for a BUD. I stated there not to use a BUD for KU because I knew most people wouldn't go to the extent to make it work correctly, especially when offsets are so much easier to install. But if the attention to detail is worth your while, some BUDs can be made to work great as a KU only dish.