I'd make the pieces as large as possible. Seams add weight.
How about if you back it with flannel and don't use batting? That might be a way to minimize added weight.
I know you didn't ask, but the thought occurred to me that contrasting colors look good on denim. Orange is the classic color used on jeans; I made a denim coat with lime-green stitching that I thought looked good with it. (Just my $.02.) Also, what about mixing the denim with khaki (if available)? Still durable but a bit lighter in weight.
I'm keeping my eye on this thread because I have dogs and a "working quilt" would have to be tough enough to endure their efforts to make a nest out of it.