For a lighter client you will have to put some time there. So.. you have a server like 5th degree ? (example..) Delete all the items that are degree >5, from npcs, from drops, from everywhere.. including itemdata files and so on.. I've noticed that such big changes could cause many crashes in the client so be careful. Do only small edits at once, so once it goes wrong you can reverse and start over again until you find a solution on how to make it work. All the not needed stuff should be gone. Also Data.pk2/Media.pk2, once you build them (they have all the files you need inside) do this:
1. Fully extract the files, using a normal extractor ofcourse.
2. Create an Empty pk2 file using joymax tools
3. Import Data folder into the empty Data.pk2 (it should be 4KB while it's empty)
4. Import Media folder into the empty Media.pk2
This usually reduces size of the pk2 files, because importing too many times into these files sometimes increases their size and sometimes not (excuse me but on that question I am totally lost and can't explain why it happens
)
Have fun