No, what I understand you to be saying is not normal. As I understand your question, you are installing from game disc to the WBFS-formatted HDD/partition via the Wii DVD Drive using USB Loader GX. All your games are showing in WBFS Mgr as using 8.45GB. Not even if all three of your games were the dual-layer games would they all have identical sizes.
SSSB-6.98GB; I don't yet have the size for the Metroid Prime Trilogy as that is dual layer too. I don't have an explanation for your discrepancy except that sometimes the files get screwed up in display-the game files are often just fine, just the display of data as I have seen occasionally. Sometimes it just goes away. As I said, I do not know the reason for this, but is is not seen very often. Luck you, right? I hope the games are playing alright for you, though.
Perhaps either use a different revision of USB Loader GX or try a different USB Loader altogether to see if anything changes? Here is a link in case you want to explore other USB Loader GX revisions:
[Only Registered Members May View Links| Click here to register]. The channel wad files are to be installed with Wad Manager; .dol files are to be renamed as boot.dol and placed inside a named folder and then placed inside the Apps folder. If SD card file structure as such: SD://Apps/USBLoaderGXrevXXX/boot.dol.