Keeping in mind that in the Imperium, anything that can be done manually instead of automated is done so.
So when those giant gun batteries turn it's not a machine doing it, it's a hundred men pulling a giant chain turning it (though they possably recieve some form of machine amplification).
In the Imperium suffering isn't just allowed, it is encouraged.
I like that Starship Dimension site, and those numbers sound about right to me. If anything, possibly a little on the small side of things. In recent years 40k has taken a turn towards the realistic and tech... in the old days of dark lore and gritty Brit sci fi, I think things would have been bigger... but time and the style of the fiction has changed and these days those numbers sound good.
Like most GW fluff though, very little is fact, it's almost all hyperbole or biased reporting. So whatever you write is okay (as long as you leave the relative ship sizes intact), as is anything anyone writes that disagrees with you. That's 40k fluff for you. Even when it contradicts, any source is as accurate as any other in GW's eyes.