But nevermind, I changed my NFS server from Solaris 10 x68_64 to a NetBSD 7 x86_64 and Voila ! It worked ! No more "File too big" error.
Pretty weird though..
It seems there might be something not working between Solaris NFS Shares and NetBSD /dev in rootfs.tgz or it might also be the Solaris "tar" instead of the GNU tar ( just tought of that one ) not extracting the /dev as it should.
So for now it'd say if anybody has the same issue, you might try to extract the rootfs with gtar instead of tar but if it doesn't work, simply don't use Solaris NFS for your netboot, use NetBSD or Linux ( haven't tried but I guess it should be working).