More likely you are seeing an error as to perspective. If you work in too close, the nose reproduces too large and the ears too small. Such facial modifications will be seen by the client even if they are minuscule. This is because the client has a mental picture of what he/she looks like and this is derived from the dressing mirror. Your task is to select a camera to subject distance that will deliver this mind’s eye viewpoint.
First – most pictures need not be correct as to perspective but portraiture is an exception. This is not truly a focal length thing; this is a camera to subject thing. When we compose we abhor leaving space around the portrait subject. What is needed is a moderate telephoto, a lash-up that delivers just enough magnification that we are forced to back up. If we can bring ourselves to just back up, a “normal” lens will do.
The focal length of choice is one that is about 2X thru 2 ½ X of “normal”. Now “normal” is a focal length about equal to the diagonal measure of the format. The APS-C (Advanced Photo System – Classic format) measures 16mm height by 24mm length and the diagonal of this rectangle is 30mm. Now “normal” for this size is a lens that delivers about a 45° field of view; camera held horizontal – that’s 30mm. Using the rule-of-thumb the suggested portrait focal lent is 30 X 2 thru 30 X 2.5 = 60mm thru 75mm. We are talking actual focal length, not equivalent. OK – you have a 50mm, I say that’s close enough. All you need do is force yourself to step back a foot or two.
Now comes the basis; nobody said this stuff is easy! A 50mm lens is mounted. An 8x10 inch print is made. The magnification to make the 8 X 10 from the APS-C is 12X. In other words, the 8X10 is the frame magnified 12 times. The viewing distance to visualize this image with correct perspective is focal length X magnification. Thus the ideal viewing distance if a 50mm lens is mounted is 50 X 12 = 600mm = 2 feet. Now beauty shots are placed on the wall or mantel and typically viewed from 1000mm (3 feet). Suppose you mounted a 75mm and made an 8X10. What is the viewing distance? Answer 75 X 12 = 900mm or about 1 yard. See how that works. This may be controversial stuff to some, but this is the basis of making images with correct perspective.
I suggest you don’t need to change lenses, just step back, this will yield a slightly smaller head size. You will need to increase the magnification slightly, say from 12X to 18X when you make the final print. That lash-up is 50 X 18 = 900mm = about 1 yard. That’s the secret. Now watch the other posters pounce on me!. Picked this up years ago. I was an instructor at the Professional Photographers School for continuing education. My subject was color print and process. I often help out as a gaff when the Master Portrait gurus taught -- some rubbed off.