Jérôme Pinot took the size of every Linux kernel (the .tar.bz2 package) since Linux 1.0 and through the recent Linux 3.1 kernel release and plotted it out. It's comparing the size of the kernel versus the release number (not against the time). His findings are that "Impressive, it's mostly exponential. If dev keeps same pace, we should break the 100MB at linux 3.19."
For reference in comparing the expectation that the Linux 3.19 kernel could reach 100MB in compressed source size, the size of linux-3.1.tar.bz2 is 74MB.
His findings can be found on the mailing list and the graph is on his blog (the GNUplot result is also embedded below for quick reference).