There is nothing about it on the wiki. Is it infinitably scalable.
.sif is a Windows system file. .sifz is a synfig project file. Scalability applies to neither, as far as I can tell 
Actually sif is a synfig project file while sifz is a gzipped sif file. Perhaps also a windows file, my knowledge about those are very limited.
Oh? Ok. It compresses automatically?
In any case, you can’t scale that either!
+1 for rylleman. Both sif and sifz are used by Synfig. Sif is (just) human readable XML. sifz is a gzipped version of the same.
Windows also uses the same “sif” extension for other files - such as Windows NT setup and autodesk, but those files cannot be opened by synfig, and Microsoft or autodesk cannot open synfig sif files.
Witch wiki ??? : en.wikipedia.org/wiki/Synfig#File_formats 
In one way … as Synfig use vector graphics … more precice infos from the wiki (synfig doc)
Synfig Features / Spatial resolution-independence
Most elements are vector-based, and all layers are parametrically generated, hence even when changing the target resolution of a project, the only pixelation will occur in imported raster images, not the built-in components
I suggest you to stop until you reach infinite… render may be tremendously long 
see(:ya!
Have done a little summary of this and pushed it in the synfig FAQ …
What is Synfig Studio native file format?
Synfig Studio use it’s own file format : .sif and .sifz. Sif is a synfig project file while sifz is a compressed (gzipped) sif file. As Sifs are (just) human readable XML, you can save some disk space choosing the compressed one when you save your animations. There no (knowed) link between others .sif file format or this one. …
feel free to adjust it 