Why is it?

Why doesn't size equal the number of pixels?

Why can size be a floating point number?

Why is it impossible to have labelings with size smaller than 5 pixels?

Because we now first dervice a polygon for each object and then calculate the size (and other properties) of the object. The polygon points are defined in real-coordinates and especially points lie inbetween pixels. therefore, sizes can differ from old calculations and floating/double precision. In contrast to that, before we simply used the number of pixels as the size of the object (and also to calculate other statistics).

Anyway, I agree it's not good to only offer the calculation based on polygons, as e.g. polygons may contain holes etc. So what we will add for the next release is an additional feature set where the calculations base on pixel values , like before. Like that the user can explicitly choose whatever he likes and is most suitable for his analysis.