Tower Spacing

I've been wondering if tower spacing can be an issue based on champion size. Testing different champions in-between the two middle lane towers of both sides have shown different results. In the image Tower Spacing 1, Fizz can fit in-between the two mid lane towers of red side, however, in the image Tower Spacing 2, Fizz cannot. I would like to question, is this intentional or is this a bug gameplay wise? Is there clarification between the spacing differences?