I'd bet money on very few people having actually tucked and ignored their male bits for long stertches of time so lots of the ifs, buts and maybe are probably speculation by folk who give their own bits a regular exercise regime.
Charles Atlas went from a97pound weekling to a hunk with exercise but how heavy was he when he died?

I have to have a douyble op SRS next month due to lack of use shrinkage.   that is fact.

I dunno.  I tucked consistently for more than 15 years before GRS.  I didn't tuck when sleeping but did all the time when wearing clothes.  I would push the testicles up into the pouches in my body from which they had originally descended.  Often I would wear a panty-girdle (or two) just to suppress it all as much as possible.  My spouse told me that I was "on the small side of average" but there was still plenty there when I finally got GRS.  I have good depth and once I get it started the largest dilator slides right in. 

I don't know if tucking can cause shrinkage.  My doctor said no.  My experience says no.  YMMV.

(And thank the stars that all that donor skin is now where it belongs.  )

- Kate