Zooming as implemented in modern browsers consists of nothing more
than “stretching up” pixels. That is, the width of the element is not
changed from 128 to 256 pixels; instead the actual pixels are doubled
in size. Formally, the element still has a width of 128 CSS pixels,
even though it happens to take the space of 256 device pixels.In other words, zooming to 200% makes one CSS pixel grow to four times
the size of one device pixels. (Two times the width, two times the
height, yields four times in total).
Source:
Concept of device pixels and CSS pixels