Here we can see the erection process, it is shown how penises grow to get fully erected. Showing that although big flaccid penis will probably get bigger when erected, flaccid size is not always an indicator of how big it will get.
Most studies show that smaller penises grow proportionally more than bigger ones, below we can see some examples of Size 1 evolutions:
As we have said on average this penises are the ones that grow the most, commonly more than doubling or even tripling its length. It is not uncommon to find many examples of S1 penises that grow into bigger sizes when erect:
The last one even jumps from Size 1 to Size 3.
Size 2 evolutions, on average grow from 5.5-8cm to 10-13cm:
Size 3 evolutions, on average grow from 8-10.5cm to 13-16cm:
Size 4 evolutions, on average grow from 10.5-13cm to 16-19cm:
Size 5 evolutions, on average grow from 13 to 19cm: