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These two pages are quite a bit bigger than all the others, so I cannot tell where Google's cut-off is for determining that a page is "too large" for mobiles, but it is somewhere between 21kB (Size O.K.) and 30.3kB (Size too large).
Unlike other errors and warnings I've seen, there's no icon next to the warning to click to see the documentation for this error, so I guess they haven't documented it yet.
The good news is that I limit my mobile-device pages to 17.5kB, well under their threshold... :)
We are probably on the cusp of a mobile explosion, thanks in good part to the success of the iPhone. I expect many more of us will be looking a lot harder at mobile browsing and mobile search in the very near future.