Meaning of に in この狭さに


I'm struggling to understand how the bolded part in the below sentence changes
the meaning of the sentence.

I think that 狭さ refers to narrowness(in context that they are both walking
close together in a very tight space), but I am not sure which に this is and
how it is modifying the rest of the sentence.

> 七菜海のリアクションが、可愛くてそれでなくても、 **この狭さに** 、密着でドキドキの俺は、悟れまいと尤もらしいことを言う

