Well the videos aren't pre-rendered, since you can customize your performers. They're all in-game and it looks pretty good, so I guess they had to pull down the resolution to get it to run smoothly.I am talking about Miku. Higher res videos can get quite large (obviously), upping the res for 30+ music videos would've made it go above 4gb, I'd imagine.
It's not that noticeable while playing though, but it looks so bad in menus when you see your performers model. Thankfully they amended this in PDF 2nd, where it's apparently sub-native while playing, and native in menus.
totp: buy the hatsune miku games while on sale wherever you are