A galaxy is described as a quadrant consisting of millions of galaxiess. So in DBZ, "Ginga" means quadrant:
エリア
Area
Means each region of the universe, which is divided into East, West, South, North. It is expressed as the "East Area" and the "West Area." The areas divided into East, West, South, North are governed by respective Kaiou.
[Par.] Like the universe, the Demon Realm is also divided into multiple areas.
銀河
Galaxy
A gathering of local planets in the universe. Planets gather and form a nebula, and beyond that, a collection of gathered nebulae is called a galaxy. The four Kaiou who rule over the east, west, south, and north [sections] of the universe actually govern these galaxies. Because the Kami in the Heavenly Realm and the Kaiou supervise the galaxies that exist infinitely in all the universe, the sections known as the East, West, South, and North Galaxies are denominations that came into use through their duty.
http://magikarp46.com/dragonball/guidebooks/07-worldview.php
The Universe
宇宙/Uchuu
Translation: the universe, space, outer space
The universe is the upper portion of the living world, and is the realm where Goku and the other main characters live and die. It’s the only portion of the DB world that even remotely resembles reality, being based off of the actual universe (or at least the universe as seen in countless sci-fi space operas like Star Wars). As such it consists of countless planets, stars, and other celestial bodies. These 4 galaxies serve as the governing areas for the gods, and so each one is overseen by the corresponding Kaio and Kaioshin. In fact, Daizenshuu 7 says that the “galaxies” of DB are merely administrational units used by the gods
So according to King Kai, Broly almost destroyed the ENTIRE southern quadrant (A couple of planets is nowhere near the size of a universal quadrant):
It would sound retarded for Broly to destroy one galaxy and then go the northern galaxy when their goal is to destroy everything and enslave people.
Movie 8 Guide states this:
"who's been laying waste to the South Galaxy."
So Broly destroyed 1/4th the universe over time, which is a ridiculous feat indeed. Even in Broly's life span (as old as Goku), he is going to destroy the entire universe:
And the director of Movie 8 states that Broly can destroy
GALAXIES:
Shigeyasu Yamauchi : "When I had to work on the Broly movie, I realized that he lacked something, although he is the most powerful Saiyan. It's this solitude of the powerful warrior that I wanted to retranscribe. He can destroy galaxies but is alone compared to Goku, whom he despises but has a family, just like Vegeta. When he loses at the end, he realizes that strength is not just physical, he would have been able to unlock another kind of power if he had something to protect for example."
http://www.kanzenshuu.com/forum/viewtopic.php?t=42231
And if it was a single galaxy, King Kai would've knew directly that there was someone recking the galaxy, but he only knew that after the Southern Galaxy was almost eradicated.
Broly = Multi-Galaxy level > Super Perfect Cell. Pretty much intentional kek.