I agree that you need to taper the finial a bit. I don't think the barrels really need to be bigger. Maybe the upper barrel a little, but I think the lower barrel diameter is fine. If it were me, I'd have a slower taper from the nib. It seems to get fatter fast then level off. I'd just soften that a little. Did you undercut the upper barrel so the lower barrel will go into the recess? It looks like you did, but I can't tell for sure. I think it gives the pen a cleaner look. I didn't do that when I first started doing my own centerbands. I can sure tell the difference. I think what would be really cool would be to make the centerband and finial a solid red or blue. If you haven't done that before, just cut a tenon on the upper barrel, then drill and square a scrap red piece (if you have one), and glue it to the upper blank where you cut the tenon.
All in all, a very nice 1st effort. One year I made all my pens for the troops with fake centerbands. Some had custom finials to. It was good practice.