Actually... it's legit to do this. Bernie Brewer pointed me to a section of the NEC that describes "feeder taps" when I was asking him about whether I could or could not do this sort of thing.
As long as the smaller gauge wire immediately connects to an over current protection device that is properly sized for the smaller wire, it's A-OK to do. There are other rules related to whether the feeder wire is or is not running a long distance or whether it needs to travel through conduit and other stuff, but for short jumps within a single enclosure, most of the rules don't even apply.
I was very thankful for that, too, because I had 50A coming in and wanted to drop down to 25A for my element and 15A for my controls, and the thought of running a big thick-ass wire from the distro block to the panel where my 15A breaker is installed did not thrill me.
