I'll try to answer what I can.
Supposedly the attacking force was in the 'millions', but by contrast a Hive supposedly has populations in the multiple billions.
There were millions of Orks outside of one
hive city sometime in the first
day of the invasion...this is while the Imperial Navy was still fighting over the planet and the orbital defenses were still destroying most of the approaching Ork ships.
God knows how many Orks eventually landed on the planet. But it is the largest Ork invasion of an Imperial planet...so hundreds of millions at the bare minimum, probably.
True not everyone is a soldier, but the harsh realities of daily life in a hive complete with gang-wars and constant warfare and such would mean the at by in large most citizens would be armed, and tougher than your average joe.
From the parts that we do manage to see of Helsreach in the novel (admittedly not a ton), it seems like the hive is relatively civilized. I can imagine that there's been some pretty serious heavy-handed martial law on the city after the second war. From the scenes dealing with citizens (non-militia civies), they're an armless lot that shys from fighting.
The galaxy is a big place. Apparently Helsreach doesn't have the same sort of day to day brutality we've seen in other hives.
Also shooting up a shopclerk is very different than fighting an 8 foot green killing machine.