Content length limit i admin page?

A friend of mine that I helped setting up Grav seems to hit a problem with page lengths. He has created a very large markdown file size 70kb (that's about 22 pages of text). I had converted that from his older CMS and uploaded it via FTP. That works and the page shows nicely on the page. But he can't edit it as it won't save with the error that the content is not correct. I tested and found out if I remove some text until the file is 65706 bytes., it does save fine. But if I type one character more, the error shows up. This happens both on the hosting server as with grav server on my laptop. So it is not a limit on the hosting provider.
Question: Does Grav have a hard limit for the content length? If so, can I change that limit? If not, is this a bug or should there be something in my config?