You're not convincing me that 15-20 MB is a problem.
My first personal computer had 64k of memory, only half of which was available to applications. There are still a few of these around and working, but I don't bother constraining my applications to run on them.
Once again, the answer at the end of the day is simple - you're out of luck. Webkit isn't modular, and doesn't allow you to make use of subsets of its library. If Webkit's size is a problem, you'll have to find some other way to do whatever it is you're working on.
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