I'm trying to work out if it is actually desirable to separate the larger projects onto one thread. The only way you can have a smaller project dumped more often is the have the larger ones dumped less often, but do we really want less frequent enwiki dumps? By separateing them and sharing them fairly between the threads you can get more regular dumps, but the significant number is surely the amount of time between one dump of your favourite project and the next, which will only change if you share the projects unfairly. Why do we want small projects to be dumped more frequently than large projects?
I guess the answer, really, is to get more servers doing dumps - I'm sure that will come in time.