It isn't all that surprising that the size of the halo is as large as it is.
Firstly we consider the way that galaxies have grown over the last 12 billion years, through collisions.
Although it is theorised that there were actual galaxies formed in the early universe when unusually large amounts of gas coalesced, a large portion of what we see today has grown up through the
Bottom Up model which first starts with dwarf galaxies and globular clusters (consider them unevolved or stripped dwarf galaxies).
As galaxies form via mergers and although there isn't much in the way of stellar collisions it is still a very violet process where vast amounts of stars and gas to get flung out of the newly forming galaxy. Due to the enormous gravitational well that a galaxy has, this material doesn't entirely disperse into the deep black yonder; it forms a halo.
In the past most of the research into
x-ray halos has been on galaxy super clusters where there is vast amounts of mostly invisible mass between the galaxies in that cluster.