Thanks for your help. I was contacted by my hosting provider and he explained me that there was a shell restriction and he removed this restriction.
It was not related to memory_limit.
I think this page https://support.cpanel.net/hc/en-us/community/posts/19163447287703-Max-Memory-for-a-User can give more information about the problem I encountered. As a shell user, I was limited to 200M of RAM for a process so even if I was increasing memory_limit it did not solve my problem.
Hope this can help other users encountering the same problem.