Basically fast CPUs need more cache because cache is what lets them run at top speed without waiting for slow(er) RAM to catch up. So the bigger the cache, the less the need to wait on RAM. But the opposite is also true ie if its a slow CPU then its not really waiting on RAM as much as waiting to calculate stuff so cache size, beyond a minimum, is not gonna help.

From your tests it seems that the 1800 just happens to have 1MB cache as a 'perfect' choice for it  and is too slow to need any more. As you go for faster CPUs and add more cores, this need will grow all the way to where you need 8MB in a quad core running at 3GHz.