dlm4
That is normal and usual - inner tunica expands while erect, and there’s less room for outer tunica to stretch, so to say. Some people do have larger erect length, but that’s due to long glans.
I think I’ve settled with a set of preparative exercises:
Simple twists, to left and to right, one minute each, one time.
Stretched bends - 90 at base, 180 just below and above midshaft - to left, right, up, and down - 30 second each. Total of 12 bent stretches.
Isolated twists - base, midshaft, vertical and horizontal grip, left and right - total 8 twists, of 30 seconds.
All these done fully flaccid. My guess is that way they’re more length-focused, you can also go them plumped flaccid - then they would be more girth-focused. Plus, for girth, O-bends should be added.