Pretty much, yeah.
It's certainly hard to see how Labour can end up not losing more than the Tories. They're currently in power, and look pretty likely to lose it. The Tories aren't, so can't lose what they don't have.
It's also often the case that the incumbent loses elections, rather than the opposition winning them. As a species, humans (IMHO) tend to go with the flow, to resist change. There's an inertia factor that supports the incumbent initially, and certainly while things are going well. It kept the Tories in power for a long time last time, and kept Blair winning elections too. But as soon as things start going badly, or when governments start getting arrogant and complacent in their power, the pendulum swings. Labour will take the blame for a whole series of things, from resentment over ID cards, to the drip-drip corrosive effect of losing our data on CDs, to the 10p tax farce, to the Iraq war, to posin emails from Brown's own staff, to .... and so on. Individually, probably none of these would lose elections, but when they seem to lurch from disaster to disaster, the cumulative effect, sooner or later, is terminal. I think Labour have well and truly tipped over that the now. I think they almost certainly had before this expense scandal, but that'll do for them.
Unless something unexpected crops up between now and the next election. But for that to happen, it needs to be something pretty seismic to devastate the Tories.
And another thing. The way I'm reading it, most Labour MPs have pretty much accepted that the next election is pretty much lost. Most of them won't say that publicly (though a few have), but many do privately. That means that individual Labour MPs are now going to be going into butt-saving mode. They are going to be more worried about their personal situation than the party situation, because it's looking more and more like they can't rely on getting elected because they're members of a party that looks likely to lose. That means they're likely to start growing a pair of gonads and standing up to party whips, like we saw with the Gurkha vote. The result is a considerable weakening in Brown's position and authority, and he's looking like he will need to Tory support to get some policies past his own rebels.
Cameron, on the other hand, benefits from a party that, whatever their personal opinions, know that their best chance of not screwing up their chances of getting into power reside in presenting a united front. Ask yourselves why Ken Clarke is biting his tongue over Europe? He seems to have acquired a serious case of dontrocktheboatitis.
And finally, yes, Cameron has looked decisive, like a leader, over this expenses business. When things came to light, he jumped on his own party hard, and immediately. Brown, by contrast. seemed to be dithering, trying to work out which committee to commission a report from. And when he did finally get going, it was after Cameron and Clegg, and looked v ery much like a "me too" afterthought that had been dragged out of him because the others had acted. Regardless of the facts of it, that's how it looked.
Finally, in regard to who appears to be most damaged by the expenses thing, well, I haven't done any statistical analysis of which party seems to have it's members snouts most firmly in the public though, but my gut feeling is that it looks to be Labour. There has certainly been a number of incidents on both sides, but my sense of it is that Labour have been much more seriously exposed to it, both in terms of the size of the typical abuse, and more especially, in the number of high profile MPs (i.e. Ministers) facing accusations.
Certainly none of the main parties can hold their heads up high (which accounts for why they're not lobbing accusations at the guilty), and of course the revelations aren't over yet, so the situation may not stay as it is right now, but currently, that's how it feels to me.