Hi FeralJim,
I'll try and help you understand things a little easier (as a whole and not just with WoT).
When dealing with software such as games, apps, etc you have two types of folders that get used (among the others) they are /assets and /res (which is short for "resource" and where authorized resource files go. They can be .avi, .xml, .ogg, .mp3 etc).
The way WoT and other games work to keep things clean, is, you create a separate folder for mods (when used) with a resource sub folder in their so you don't over-write the main game/files. If the game sees those files there it will read/use those instead of the main game files.
When a mod pack such as Aslains comes along, they make it easier and help keep things clean and organized within those sub folders and do not over-write the main files of the game.
In order for Aslain to use mods in his installer he has to re-package them and place them on his server to be used when called upon by his installer. He likes to use 7zip which has a higher compression ratio (and better in my opinion). The server is *nix based (Unix/Linux) which has a different file system (files table) than Windows so the file sizes will appear different because of how they are read even though the file is the same. (You have have heard the terms NTFS, FAT32, etc, those are File Allocation Tables and read/write files differently than each other).
I hope that helps a bit.
-CS