The s3fs project that you've linked to provides a really bare-bones FUSE filesystem for s3 that is missing many features. It's not POSIX compliant because it's lacking lots of POSIX features (such as hard links). It also uses some cheap hacks such as, whenever any file is to be written to S3, it writes it to a /tmp file first and then uploads that. This limits the files you can write to S3 by the size of your /tmp partition and also introduces the possibility of random strange errors in writing to S3 that are due to out-of-disk space issues locally. Also, the s3fs code quality is not good.

I was hoping to do something much more complete ...

The host I'm on has more free space than that. Maybe your coincidence is due to the fact that both hosts are full?

(18 x 1024M x 30) + 137040M = 690000M

It's a nicely rounded number which we don't often see in computers, so the limit is likely to be by design. And it's just a few gigs shy of the usual capacity of a 750GB hard drive