For reference, my 4x10 is about 10 cm narrower than the JVM410 head. It's not much, but enough to make it look a bit odd on top of it.
4x10s are a classic case of "nobody famous used them, so they're not popular." The Super Reverb and Bassman, as used by SRV and others, remain basically the few somewhat popular 4x10s.
Yes, compared to many other amps there's plenty of space, and I can bet that amp could be designed to fit into even less. They just didn't need to do it when they knew it was going into this big box form factor.
If they ever make a successor, I'd love to have it be a bit smaller and lighter.