An Anvil trunk will not be smaller than a Walt Johnson case. Instead of those nice rounded edges, Anvil builds a box, making it even bulkier. In addition they are very heavy. I had one for my Getzen G-50 that was 105 lbs. with the tuba in it.
I would not suggest Anvil as an alternative to Walt Johnson for the reasons you have. You'll end up with an even bigger, heavier case that you have now.