Brunk?!? that is the first time I've heard it called that... and its perfect Bed + Trunk = Brunk
I think the reason its not deeper is the spare tire is UNDER it which is better then Ridgeline where you have to unload most of the bed (and maybe even the brunk) to get to the spare out. That would be super frustrating if not impossible under some conditions.
I remember when I got my first Miata and there was no spare tire, just a can of Fix-A-Flat and I freaked out at first. Never had to use it (but I did have to replace it with a new can periodically...and with AAA roadside service, never gave it a second thought. I now carry a package of tire plugs and an inflator with me. Seems like 90% of the tire issues are within the fixable area, so why worry?