BootstrapperApplicationData.xml contains correct InstallSize for MSIs, but not for MSPs. Is it possible to compute it for MSPs?
<WixPackageProperties Package="Id.Simple.Msi" Vital="yes" DisplayName="Simplest ECO kit msi" DownloadSize="32768" PackageSize="32768" InstalledSize="51" PackageType="Msi"/>
<WixPackageProperties Package="Id.Simple.Msp" Vital="yes" DownloadSize="24576" PackageSize="24576" InstalledSize="24576" PackageType="Msp"/>
Its very difficult to predict the installed size of a patch. It depends on which products it targets, which ones are installed on target machines, which version of Windows Installer is on the machine, the state of the baseline cache, the type of update, etc.