The fact is that the many binary under /rescue should have been hard links to /rescue/id (that's the way FreeBSD ship them in their base.txz, in our ISOs those links doesn't exists anymore, and have been replaced with a copy of the file.
The result clearly is an unneeded huge /rescue.
I'm currently investigating the problem, although I suspect it was introduced from when we started fetching kernel and base.
FreeBSD base:
$ tar tvf base.txz | grep rescue
drwxr-xr-x 0 root wheel 0 Nov 11 2014 ./rescue/
-r-xr-xr-x 0 root wheel 8022232 Nov 11 2014 ./rescue/id
hr-xr-xr-x 0 root wheel 0 Nov 11 2014 ./rescue/nc link to ./rescue/id
hr-xr-xr-x 0 root wheel 0 Nov 11 2014 ./rescue/echo link to ./rescue/id
hr-xr-xr-x 0 root wheel 0 Nov 11 2014 ./rescue/kldstat link to ./rescue/id
...