>>29039807sources or made up
the only study i could find on the subject is this meta-analysis:
https://wjmh.org/search.php?where=aview&id=10.5534/wjmh.220203&code=2074WJMH&vmode=FULL
which claims a 29% increase in length in 29 years.
and theres also this penis size awareness site which covered it quite critically:
https://bigdickguide.com/stanford-study-wrongly-says-penises-are-getting-bigger/
which points to many failings of the papers the meta study cites.
And as the article points out, many of the cited studies have misleading information or dubious details:
5 studies take flaccid stretched lengths, and most of them from the 80s and 90s, as realisation that bone pressed method yields more consistent (and greater) measurements is recent.
2 studies measured shaft length, excluding glans.
4 studies were from groups of people with impotence, seeked penile lengthening procesures, or otherwise had medical conditions that might affect penile size.
Most absurdly, 2 studies took measurements from cadavers, one of which, which also featured the biggest numbers in the study, were surgically removed, frozen and thawed before measurement.
And a couple of other studies with blunders such as not mentioning wether measurements were bone-pressed or not, used self-reported data, even instructing participants to measure from the underside of the penis (where there is no bone to push against).
Even without all these studies removed, scientific integrity of the paper is questionable at best, but even wih the remaining studies, the picture of a size increase is very murky.
As for why you see more big dicks in modern amateur porn, its almost certainly because more wild-west approach of the internet provides for natural selection: while porn industry is based on close ties and rigid contracts, popularity on the internet is almost solely dictated by the attractiveness of the actors, which also includes penis size in men.