It is a general thought so I hope it make any sense, it the case of a function $f:\mathbb{R}\rightarrow \mathbb{R}$ we could talk about onto and isomorphic map. is it the same with $g:\mathbb{R^2}\rightarrow \mathbb{R}$ or $h:\mathbb{R^3}\rightarrow \mathbb{R^2}$
do not we "lose" data going from an higher dimension to a smaller one?