Comment # 8 on bug 1104588 from Bruno Friedmann

Some complement on default Leap 15.0 /tmp is not a tmpfs file a subvolume @/tmp
is created and the mount is done there.

Now to have what the administrator think it is better for its own usage
the tmpfs module in the partitionner can as before add a line in fstab, or
create the desired configuration file automatically by surcharging the default
For example 

/etc/systemd/system/tmp.mount.d/options.conf
[Mount]
What=tmpfs
Where=/tmp
Type=tmpfs
Options=mode=1777,strictatime,nosuid,nodev,size=2G


reference

https://unix.stackexchange.com/questions/352042/systemd-backed-tmpfs-how-to-specify-tmp-size-manually

With this kind of behavior, is the most sensible world. be really systemd aware
system, and offer tuning (as we always being strong) to sysadmin.