It gets better once you understand that the Earth forces in RayStorm are meant to represent the Roman Empire of antiquity, and Secelia an aggregate of Phoenician Carthage, Vandal Carthage, and the Visigoths. It seems that one of the big (and repeated!) gaffes of the late Roman Empire was leaving Rome itself understaffed with soldiers. The reasoning seems to have been that the legions were needed more at the empire's frontiers, and anyway, if invaders couldn't get through that (and they usually couldn't), why would Rome itself need to worry? Unfortunately, as Honorius found out, that doesn't always work. For one thing, Alaric had previously been a mercenary in Rome's employ, before he became a leader of the Visigoth conquest; he'd have a pretty good understanding of how to navigate the territory and roads near Rome! (Help from disaffected slaves, like opening a crucial gate into Rome proper, didn't hurt.) Important point is, the early backstory battles between Earth and the exploited colonies can reflect the Punic Wars and the early Caesars' campaigns, while the in-game present is more like Rome being embattled by "barbarian" invaders out for both wealth and punishing the impious (the Vandals and Visigoths were Arian Christians, and so wouldn't have had a terribly high opinion of the Athanasian Romans and their seeming blasphemy of giving some of the glory due to God to Jesus instead, even if Genseric and Alaric went no further than emptying Rome's treasuries).
And yes, from the time of Alaric onwards, Rome quickly dropped to a fraction of its old population. After Alaric emptied the treasuries (and other than a select few burned buildings, that's all the Visigoth sack really entailed), Rome's internal economy was in a bad state, forcing many denizens, nobles included, to leave the city for greener pastures. And of course, there was the unhealing wound to Rome's morale inflicted by being successfully taken after EIGHT CENTURIES of being untouched by foreign armies. Of course, this is where the history/RayStorm parallels shift. While Secelia's success would parallel the Goths, what they plan to do with Earth after forcibly evacuating the population (read: binding them into slavery?) is more like what Rome did to Phoenician Carthage after the Third Punic War--razing the city, salting the ground, and all that. (Maybe that's what Carthage would have done to Rome if IT won out? Hannibal was Carthage's premier general in the Second Punic War, and the planet-killing battleship is named after him...) There may also be a bit of the popular understanding of Alaric's sack--a festival of mayhem, pillaging, and Unseemly Things far removed from what actually happened. Think in terms of what "vandal" usually suggests nowadays; it didn't have that connotation before the 1800s at least, I think.
So "normal" Arcade and Extra perhaps represent Rome in its more cautious days, the Republic and the early Imperium, knowing better than to send literally all their forces out. 13-Ship...late Empire methodology. Complete with final twilight.