A few days ago I took a look at a thread about the population of Sirius, and I was slightly surprised. I expected Gallia to be a hugely populated House, but then I saw that it only had 20,300,000,000 planetary citizens. Those who live in space can be disregarded, as their number is insignificantly low in comparison.
Bretonia has 15,700,000,000 planetary citizens, while Liberty has 22,700,000,000. According to this, an economy of 20,300,000,000 is fighting an economy of 38,400,000,000, if we disregard the Council, the Crayterians and the Outcasts. Additionally, history has proven that totalitarian states have weaker economies than liberal ones.
However, Gallia had 500 battleships total before invading Sirius, 150 of which were sent to Sirius, while Bretonia had 30. I suppose other ships' numbers are roughly proportionate. But the same part of the lore (I think it was the readme of 4.86, not sure) I've read said that Bretonia was, at the point, the richest House of all by treasury, while we all know that Liberty was always the strongest economic power (but Bretonia didn't spend so much on its military or who knows what, I suppose).
Furthermore, Gallia have used up the last of their resources on building their armada, and all they have right now is what they conquered in the Taus and what little they are getting by partially open trade. Their forces are spread out and their supply lines are long and thin, while they rely on unstable jump holes. According to this, Gallia is but a mere information spinner, and they only appear powerful because of their military which can't regenerate. This pretty much reminds me of the Pacific war. Japan had a military only rivaled by Russia, Germany (its ally) and Britain (who were losing to them), while the US armed forces were the 16th in the world by their strength. On the other hand, Japan's economy was very weak compared to the US, and the stronger economy prevailed in an extended conflict.
So, if Sirius manages to prolong the war, Gallia is destined to lose. In fact, it is Gallia that is ganked here.
I have a feeling that the main reason why Gallia started the war is to calm its people with short lasting victories, but long enough to outlive old (by the look of him) King Charles XI. Après lui, le déluge.