"I'm petrified but thats no excuse to be rude", fear of the unknown always makes people act their worst, that line really shows what a good girl Riko is.
I am glad that the obligatory hamster and banana image is here. @crackkid
Basically contrast, the big guy makes her look smaller and the small girl makes him look bigger.
The other side of the coin is that men are commonly attracted to smaller women and vise versa do to some monkey brain shenanigans. Manga, as always, exagerates these traits...