Minidvd VS HD camcorders?

I recently saw these $1000+ HD camcorders at the store and I was wondering if someone could clear something up for me. When I take my Minidv Sony camcorder, which I bought last year for $400, I get a 15 gig raw movie file. Isn't an hour of HD content around 16 gigs? So technically that 15 gig raw trasfer of video is 1080i or HD right? If it isn't and these new "HD" Camcorders can record in 1080i, how come they can still use a Minidv to do it? Or How can you record a hour of video at 1080i on the same Minidv where a normal minidv camcorder can't on the same tape format. If someone can clear this up for me I would appreciate it.... thanks in advance...