pyro (WiX) delta patches are too big (when compared to bdiff) - wix

Question

I added the -delta switch when building patches with pyro (WiX) recently which seems to improve sizes somewhat. However when comparing diff sizes to those of our old bdiff-based patcher some files still come out much larger than expected. I delved into the source and it seems to be using mspatchc.dll
It appears to be one file in particular that's causing problems: The original and new versions of the file are both around the 100MB mark. Using bdiff generates a diff of 15KB, but pyro -delta uses 18MB!!
Why is this? Is it a bug in WiX? Is there a way i can improve (reduce the size of) the diffs generated by pyro?
candle patch.wxs
light patch.wixobj
melt ..\old\project.msi -out old.wixpdb -pdb old\project.wixpdb -x old_bin
melt ..\new\project.msi -out new.wixpdb -pdb new\project.wixpdb -x new_bin
torch -p -xi old.wixpdb new.wixpdb -out diff.wixmst
pyro -delta patch.wixmsp -out patch.msp -t proj1 diff.wixmst

Answer 1

I don't know if this should be an answer really, this is just an educated guess.
I'm guessing bdiff creates diff files of only the differences of files and has special logic to apply those diffs to the installed files. Alternatively, pyro -delta includes whole file if they are different.
You could test this by including a gigantic text file and then using bdiff after changing one word to get a tiny diff and then using pyro -delta will probably get you a diff basically the size of the text file.

Answer 2

This is more of a work-around, but by using more fragments this issue can be mitigated (at least until you need to patch the file in question). For example:
<Fragment>
<DirectoryRef Id="MyApp" DiskId="1">
<Component Id="MyApp.exe" Guid="*">
<File Id="MyApp.exe" KeyPath="yes" Source="$(var.OutputDir)\MyApp.exe" />
</Component>
<Component Id="Library.dll" Guid="*">
<File Id="Library.dll" KeyPath="yes" Source="$(var.OutputDir)\Library.dll"/>
</Component>
<Component Id="HugeLibrary.dll" Guid="*">
<File Id="HugeLibrary.dll" KeyPath="yes" Source="$(var.OutputDir)\HugeLibrary.dll" />
</Component>
</DirectoryRef>
</Fragment>
If HugeLibrary.dll always creates big deltas, we can move it to it's own fragment which means that it will only be in the patch if it's explicitly coded in:
<Fragment>
<DirectoryRef Id="MyApp" DiskId="1">
<Component Id="MyApp.exe" Guid="*">
<File Id="MyApp.exe" KeyPath="yes" Source="$(var.OutputDir)\MyApp.exe" />
</Component>
<Component Id="Library.dll" Guid="*">
<File Id="Library.dll" KeyPath="yes" Source="$(var.OutputDir)\Library.dll"/>
</Component>
</DirectoryRef>
</Fragment>
<Fragment>
<DirectoryRef Id="MyApp" DiskId="1">
<Component Id="HugeLibrary.dll" Guid="*">
<File Id="HugeLibrary.dll" KeyPath="yes" Source="$(var.OutputDir)\HugeLibrary.dll" />
</Component>
</DirectoryRef>
</Fragment>
All files will still be in the installer, and the component group(s) will remain the same.
<Fragment>
<ComponentGroup Id ="myapp">
<ComponentRef Id="MyApp.exe"/>
<ComponentRef Id="Library.dll"/>
<ComponentRef Id="HugeLibrary.dll"/>
</ComponentGroup>
</Fragment>