Your comments

I know. But at least the file size for files > 2 GB was correct.


I have to come back to the problem again. 

Recently I had a problem with my server and had to temporarily install a backup with php5. Surprise: File sizes over 2 GB to 4 GB were displayed correctly! For larger files, the size was given as the part over 4 GB. 

So for the span from 2 GB to 4 GB it seems to be a problem with php7. 


Sh...! You are right. It's a 32 bit system and occurs, if files are larger than 2 GB! BTW: Is there a easy way to swap to 64bit (system ist Debian Stretch on a Pi2)?