Hiyyo! We're almost the same proportions, I'm 5'3", 95lbs..don't think I'd carry a 40 because of the sheer weight of the bag but....
Hey, if you have the confidence to rock a 40, go for it! It may be heavy but if you're willing to grin and bear the weight of the bag for the sake of being able to carry your essentials, then by all means go for it!!
Besides, look at the Olsen twins(babebei also mentioned this). They're small and they love carrying giant bags. It's whatever you feel most comfortable with. People are always going to think this or that but hey, if you're happy toting around a 40cm, go for it hun!