Do Manual de Políticas da Debian :
5.6.20 Installed-Size
This field appears in the control files of binary packages, and in the Packages files. It gives an estimate of the total amount of disk space required to install the named package. Actual installed size may vary based on block size, file system properties, or actions taken by package maintainer scripts.
The disk space is given as the integer value of the estimated installed size in bytes, divided by 1024 and rounded up.
Eu suspeito que a intenção é que esse campo seja usado por gerentes de pacotes e afins para dar ao usuário uma idéia de quanto espaço em disco o pacote instalado ocupará.