Everyone has a different opinion of what is "bulky".
Zoogue: This is my personal favorite and the one that ended my case searching. This is my daily case. Its thin and protective. From other forum members (such as scjr) who had a real drop experience, sounds like the Zoogue holds up well. The ribbed sides *really* helps me with grip. Of the cases I've own, only the CandyShell Grip is more grippy. Due to the thinness and ribs, I'm very comfortable using the 6 plus one handed. The sides is a little loose if you compared it to some other cases (especially the CandyShell Grip). I've also had an old grey one close to the when the 6 plus first case out and the buttons are a bit stiff. I've recently ordered a blue one and the buttons are much better as scjr mentioned too. The lip is high enough to have some space even with temperted glass on the screen. Overall, this case is a really good balance of thin and protective and my personal favorite.
Speck CandyShell Grip: I really like this case until I got the Zoogue. Its my most protective case and very very rigid and sturdy. The grip is awesome. The buttons are the most responsive of the cases I have. The main drawback to me is the bulk. Personally, this case is too bulky and heavy. But personally, the case is uncomfortable in most of my pants pockets (too tall, especially when I sit) which is the main reason I stopped using this. The grip is a double edge sword as sometimes my pocket lining gets pulled out too. Despite the size, the grip is so good to me that I'm confident when using the phone one handed (even more confident than the Zoogue). The other nitpicky reason I stopped using it was the glossy plastic feels gross to me after a while. I which they used a matte surface. While it sounds like there's too many negatives from me, actually I still really like this case.
Magpul Field Case: Its protective and has responsive buttons. Size is slightly smaller than my CandyShell Grip but still really bulky to me compared to the Zoogue. The main surprise to me is compared to the other two, its the least grippy. The ribs didn't help that much in my hands and since its almost the size of the CandyShell Grip, I don't feel confident using this phone one handed. The texture, while matte, is pretty slippery to me. I'm not sure if I got a knockoff Magpul or they're all like this since others have a good grip with this case.
Personally now, if I had only one case, I'll go with Zoogue due to the combination of thinness (I value this the most), grip and the protection. CandyShell Grip is a second if I want a really really protective case.
Otherwise, you can't go wrong with any of them.
