I've recently switched to vibram five fingers and have started developing calluses under my big toe. Is this normal or is my strike wrong?
I have had a lot of experience with the vff (3+ years personal) and from what you're telling us you most likely have girly feet that have never had a chance to develop calluses. Get barefoot more often and as long as your calluses aren't ripping off you're fine.
Look at the bottom of anyones foot who runs a lot and you will notice both the outside of the big toe, midfoot and heel are pretty callused up, something to be proud of, not worry about.