It seems somewhere between fit and flare and sheath to me...
I think this dress looks great on you and I don't think it makes you look heavier (than other outfits I have looked at of yours) but of course, you have to feel good in it. Too bad about the armholes, especially since otherwise, it looks to fit you well in the bust, I don't even find it work-inappropriate (but again, that's up to you). Usually with this type of dress I'm one size on top, another on the bottom, that doesn't seem to be an issue here.
I didn't try sheaths for years but as I age and get a thicker waist & relatively smaller hips, I am hoping to find more that work. I have one passed on to me that is great. It has some waist definition detailing which maybe makes it a little less sheath but it functions as one for me.
Let us know what else you try!