Volition has stated that artificial subspace jumps require gravitational wells.
The Shivans (mainly in FS1) have been seen making jumps that seem impossible to then GTA and VI.
In FS1, the Shivans seemed to travel across areas of heavy subspace traffic and sterilized all worlds in their path. This meant the frontlines of the V-T war and the homeworlds of the Terrans and Vasudans.
In FS2, this was a more limited engagement, following GTVA forces into Capella where their advanced was stopped after the Capella star was destroyed and the meson bombs were deployed.
'Freespace' is another name for subspace.
All of this ties together. Assuming that the connection between subspace and mass applies on the large scale, destroying stars has some form of large scale effect on subspace, and the Shivans also have an interest in destroying any species that uses subspace technology. Or perhaps it's just neutralizing sources of temporary 'subspace distortions' which is how subspace travel would appear. Yet at the same time, permanent distortions to subspace they seem to adapt to such as the Knossos portals, and perhaps even the destruction of nodes. Seemingly taking the path of least resistance to small scale changes.
At the end of FS1 the voiceover person believes that the Shivans can rebuild nodes.
Now whether these are planned acts in some sort of massive war, protecting the integrity of subspace, or just instinct is all up to debate.
However, it's pretty clear that they are operating on a massive scale. Very massive.