Why doesn't deleting individual system files free space for an update installation?

Deleting files which are part of installed base or layered
product subsets will not produce additional free space because the
update installation takes into account that these old files will be
replaced by new versions. The disk space calculation determines how
much additional space is needed to replace an old version of a file
with its new version.

If the old version of a file is removed without removing the
entire subset in which it resides, the update installation will still
put the new version on the system. In this situation the full size
of the new file will be allocated instead of the difference between the
size of the original and new versions.

For example, if /genvmunix was 7MB and a new version of
/genvmunix was 8MB, update would need to reserve 1MB of free space
for the new version. If /genvmunix was deleted before the update,
the disk space calculation would then reserve the full 8MB for the
new file. So although 7MB was freed before the update, 7MB more
would be reserved during the update, which would result in no
difference in the amount of additional space needed to continue the

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