Just to clear up a few things - as much as I hate the /opt problem, we're talking about dev repos here. There is no guarantee about anything there - it's the wild west. So yeah, stuff should gravitate to optification, but the chief rule is still caveat emptor.
Since it's a compressed FS this is a bit tricky. There was the idea of the autobuilder inserting a helper field in the debs about the 'real' disk usage of a package (to prevent even considering packages that can't fit), but I don't know what happened to that train of though...