Existential anxiety isn't per se about the relative size of self to the world but more about the inability to grasp and control the understanding of it. Pretty much everything living strives to "control" things, and in the case of human beings this applies to mental things too. Controlling things on a mental level is usually about understanding them, not necessarily about bending them to our will.
Anxiety is tied to a difficulty in dealing with perceived lack of control (over others, over our emotions, over the future..), and existential anxiety is an example of anxiety when it comes to existence. We simply don't know enough to say something that cements control.
Thankfully one of the coping mechanisms is humour, so I'll leave the reader with the very well known Douglas Adams bit:
“The story so far: In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move.”