Is it magnetic or strap? I found that magnetic tank bags don't stick onto my tank that well!
Keep in mind that the front portion of the tank is plastic, thus won't hold magnets, but there should still be plenty of steel tank to stick to.
I have an older magnetic Cortech Elite Tourmaster Tri-bag tank bag. It sticks fantastically, but I've never used all three pieces together. For that matter, I've never used the larger bag... its just too big!
I wouldn't worry about it not sticking, nor an 8L bag being too small.