Well I just extracted the ui_1.pak then repacked it and it was the exact same size and no problem in game. Are you sure you're doing everything right?

What I did:

1) extracted pak file
2) created .bat file in the data\packs folder and entered the following:

echo ui_1.pak > ui1.txt
dir .\data\* /a:-D /s /b >>ui1.txt
xpak.exe -cf ui1.txt

3) Save file and double click the bat file.

New file has exactly the same size down to the byte as the original. Sure there's not a missing file of some sort? By the way you should have 3466 files extracted from that pack and it should be 208,824,445 bytes large.