Here's the thing, SW is full of hyperbolic statements. Lots of writers like to puff up this or that, and often in non-specific statements.
"On par" is somewhat vague. On part in what sense? It says "a massive galaxy threatening organization on par with the Galactic Empire...". That doesn't say military of the same size. It could be a threat on par, but that's relative to what's threatening.
The Galactic Empire being the biggest military is more heavily supported from more sources, *much* more specific, and just plum makes sense.
In TOR, Harrowers are the best ships ever when they come into the field. They're a step above prior ships like KotoR ships. Both by description, and simply examining them- they're bigger and badder than Revan's ships.
Harrowers are mini Star Destroyers. Less than half the size of an Imperial. 2/3rds the size of a Victory.
That says a lot about where the ships stand.
On a similar note, the Great Hyperspace War Sith Empire once threatened the galaxy. They probably had less forces than a number of the big Imperial warlords like Zsinj, who didn't threaten the galaxy, because what he was facing was so much bigger.
GalanSidenote: the amount of personnel required just to make the Death Star tick was absolutely staggering:
Not so much for a galaxy, really. A mere million is pretty tiny.
What was required to build it was rather more impressive.
Stealth MooseAnd again, population increases exponentially with time, unless the SW universe had an extinction/restart event like 300 BBY.
You can see the territory of the galaxy expand over time too. From this and this map (and to an extent, this one, which shows the absolute furthest extent of known space), there's sectors major population centers that weren't even explored in earlier times. And the empire and post-empire time were an era of even more exploration, so there's areas still being added.
I'm not sure who had more territory between Palpatine and Krayt for that reason, despite Palpatine's conquests being more complete.