I have noticed that even when drawing landscape images from photographic reference, people still get proportions wrong - mountains too high, fields too large..... I suspect this is because they are drawing what they THINK should be there, rather than measuring accurately - - and it can also be because it sometimes tricky with a small photo.
Drawing a portrait, or figure, from photo reference, is also fraught with difficulties if you simply draw freehand. It can actually be easier to use the old "hold a pencil out" method when you have a figure in front of you...because a real face is bigger and therefore easier to work with, than the tiny shapes on a small photo.
You can use the grid method to "scale up" a photo...but this can lead to fairly stiff drawings in my experience.
For a looser drawing instead, you can draw freehand as much as possible...and then use this little tool to double-check your drawing. With it, you will very quickly discover where you have gone wrong, and can then make adjustments.
For those living outside of the UK, there is the ACCURASEE PROPORTIONAL DIVIDER. For those within the UK, there is the DERWENT SCALE DIVIDER. Actually - there are lots of sources for both, and for plenty of others. you just need to check Google. You will even find instructions on how to make your own!
They all work in the same way, and this picture speaks more than a thousand words: