So this is from a trial that was reported by Shower vs. Grower: Does It Really Matter? And 7 Other FAQs, the sources for the article are listed below.
Does your penis grow more than 1.5 inches when you get erect? You’re a grower. Less than 1.5 inches? You’re a shower.
And out of the 274 reported participants, 73 (about 26 percent) of the participants were growers, and 205 were showers.
Growers experienced an average 2.1-inch change in length, and the showers had about a 1.2-inch change in length.
It states that this could likely be due to the elasticity and/or collagen of the penis with growers have more and showers having less. Also can be influenced by age.
As for keeping your penis from turtling that is simple and painless, I use an older silicone sleeve(the thicker ones off ebay) I no longer use on my vacexetnder and roll back the front and back of the sleeve so it is even thicker and put it at the base of my penis well stretching the penis forward to get it as far back as possible. Doesn’t stretch the entire penis but still keeps my flaccid longer than it typically is. If I can I also like to give myself a very light erection after extending and apply the sleeve to trap some blood up there for short while.
Gallo CBM, et al. (2014). Modifications of erectile tissue components in the penis during the fetal period. DOI:
10.1371/journal.pone.0106409
Luangkhot R, et al. (1992). Collagen alterations in the corpus cavernosum of men with sexual dysfunction. DOI:
10.1016/s0022-5347(17)36630-2
Penile anatomy - Detailed (2017).
Visualsonline.cancer.gov/details.cfm?imageid=11321
Population, male. (2017).
Data.worldbank.org/indicator/SP.POP.TOTL.MA.IN
Protogerou V, et al. (2010). Penis ligaments: Their use in "increasing" the size of the penis in penile augmentation procedures. Anatomical description in human cadavers and clinical results of a phalloplasty series.
Ncbi.nlm.nih.gov/pubmed/21090558
What do the terms “shower” and “grower” mean in terms of penis length? (n.d.).
Issm.info/sexual-health-qa/what-do-the-terms-shower-and-grower-mean-in-terms-of-penis-length/
Yafi FA, et al. (2018). Grower or shower? Predictors of change in penile length from the flaccid to erect state. DOI:
10.1038/s41443-018-0053-3