I am replacing a very old outside light fixture and have hit a roadblock. All of the fixtures I see in stores are sleek in design but mine is connected to a pretty large electrical box. The picture below shows my current light. Can I replace this with one of the sleeker more modern lights without a junction box?
No. Those "sleek" lights also must mount to a junction box of some kind. However, their box is installed flush to the wall, by embedding the box in the wall. Same as any outlet in your home, for instance.
In this case, either someone installed a wedge spacer box for no conceivable reason, or this junction box is tearing out of the wall and needs to be reinstalled properly or replaced. If that were done, this light would also look a lot better.