You can savely delete this files from time to time. The clients.met contains your 'debts' with others and shouldn't be too big. (and the BAK-file is just a back-up-copy, may be deleted too).
The known*.met files contain file hashes of all the files you downloaded once, you really don't need the data about the old ones, thus delete the files and let aMule rehash to have only the files you're really sharing inside.
But I agree about the gzip/bz2-thingy would be really great to save some more space.