Could you tell us exactly what your setup is (or what you think it is), especially whether you are using any tricks to increase app storage (partitioned sd cards, scripts like DarkTremor or apps like Simple2Ext to use them, or the Link2SD app). Also what exactly you did when you changed ROMs. Don't worry about too much detail, we need information to work out what has happened.
You say you "restored an old backup", but if you restored a nandroid backup (a backup made with recovery) of the CM7 ROM then it should be exactly like it was before you tried an unofficial(*) CM10. But a nandroid restore would also restore your apps, and yet you say that many apps are not installed. And installing or restoring a ROM won't affect SD usage at all. This is why I'm asking what exactly you did, because there are some things here that don't quite fit.
In the absence of more information, my best guess is that it might be that you were doing something for memory management before which you aren't now, e.g. using some app to move the dalvik cache to a partitioned as card. That's one way in which you might suddenly have less internal space free: if the dalvik cache was previously on an ext-format partition on your card but isn't now. This could happen if you set the ROM up from scratch, but as I said a restore of a ROM backup should restore everything. As for the SD capacity, you didn't perchance make a second nandroid backup? They don't replace each other, and I know that some of my Desire backups were getting close to a GB in size towards the end (much smaller than my current backups though - on the One the ROM is about a GB by itself!).
Edit: ah, just checked your post history and see that you were using S2E. I've never used that myself, but that does tie in with the suspicion about the partition. Check your S2E settings and see whether it has moved the dalvik cache, and indeed whether it is active at all.
I also see that you are using ROM Manager. Bit of advice: don't! Seriously, we've had a lot of trouble with that app on the Desire. At the worst point we were typically dealing with 3 people per evening who had usb-bricked their Desires, and it was ROM Manager which was responsible. It does nothing that you can't do just by booting into recovery and doing things directly from there, but it is not fully reliable on the Desire, and if you are not S-Off a USB brick is a pain to fix. There is actually a sticky thread in the Desire All Things Root forum telling people not to use this app.
(*) The last official CM7 release for the Desire is CM7, as the CM team decided that due to limitations of the hardware and lack of driver support they would not be able to provide acceptable performance with later versions on this phone. Some people have taken CM10 sources and tried to build it for the Desire, but it's not an official CM build.
