I'm not fat and I use 1 1/2" in delts, glutes, and quads. There is absolutely a benefit. That benefit is introducing the compound into your actual muscle. How much of that 1/2" do you imagine is hitting actual muscle? And unless you're injecting less than .5ml, where in the world is all that oil going?
There's also a matter of pip. The deeper you go into the muscle, the less pip you should experience. The last thing you want is some irritating excipient migrating under your skin where you are sure gonna feel it.