Comment 8 for bug 152580

Confirmed.

Workarounds:
a) resize the partition containing /, or /var/ or /usr.
or
b) mv /var/cache/apt to a different partion and make a symlink to it
or
c) mv /var/cache/apt to a different partition and edit
/etc/apt/apt.conf
with the following line:
Dir::Cache::archives "/path/to/your/archive/dir";
(see #106804)

I have a (possible) explanation why different sizes to be freed are shown.
from main.log:

2007-10-14 13:10:54,205 DEBUG dir '/var/cache/apt/archives' needs '888777698.0' of '<DistUpgradeControler.FreeSpace object at 0xa7a99ac>' (1975717888.000000)
2007-10-14 13:10:54,206 DEBUG dir '/usr' needs '658960384.0' of '<DistUpgradeControler.FreeSpace object at 0xa7a99ac>' (1086940190.000000)
2007-10-14 13:10:54,206 DEBUG dir '/usr' needs '52428800' of '<DistUpgradeControler.FreeSpace object at 0xa7a99ac>' (427979806.000000)
2007-10-14 13:10:54,206 DEBUG dir '/boot' needs '62914560' of '<DistUpgradeControler.FreeSpace object at 0xa7a99ac>' (375551006.000000)

The upgrade-manager first checks if there is enough space to download the packages to /var/cache/apt/archives. If it encounters an error here, it aborts and reports how much space is missing. If you free some space, it succeeds in this step, but fails in the next one, when it checks for space to install the packages.

If you consider this behavior a bug the installer should not directly abort after encountering that there is too little space, but go on calculating and display the total result. Or the text should be changend saying "You have to free AT LEAST foo MB of space" ...

I had a quick look at the sources, maybe I'll try to write a patch.