UV is mapped 0 to 1.
One more question, if you place images (as reference or background)
then they always have as size 5m. I scaled my reference image to 1m
for the modelling process. If I now load the same image in the UV
editor then the image isn't 5 times as big as the UV objects? Any
explanation for that behavior?
In addition to @moonboots answer, and
Leaving out the image for an explanation.
The idea of a UV is to map 0 to 1 in U and V to the height and width of the image ie UV (0, 0) is bottom left hand corner and (1, 1) is top right hand corner. Without an image the extents are square.
When using project from view it is making a square from the longest edge of the viewport and mapping that to the 1:1 of the UV.
Mapping to bounds extends such that the maximum and minima are exactly 0 and 1 for U and V.
Example square extends from top to bottom of view, but not left to right, UV result is using width of viewport to make square
Similarly if we squeeze up the 3d view to make it extend left to right, it uses the "view square" of top to bottom.
To put this another way, the size of the viewport also counts when projecting from view.