FWIW, on MacOS the installed footprint of the Sigil.app is 514,158,161 bytes (519.8 MB on disk on Intel x86_64) and 501,588,228 bytes (507.3 MB on disk on Applie Silicon arm64).
Since an AppImage is on a squashed filesystem that needs no installation, its on disk application footprint is only 213MB.
So to each their own, but a fully installed 213 MB disk footprint is actually quite good for an Application that includes its own Qt, its own Python plus tk, lxml, PySide6, etc, and of course the Sigil app itself (including all translations and dictionaries). I would bet installed on Windows it is closer to the MacOS size than the AppImage size.
Update:
In case anyone is interested here is a rough break down of the main components (in K Bytes)
Code:
Sigil.app (in K)
----------------
495416 Sigil.app
40704 Sigil itself
454712 Frameworks
328144 Qt Frameworks
87560 Primary Qt frameworks
240584 QtWebEngineCore
131612 Python.framework
17332 python3.13/lib-dynload
44980 PySide6
So fully decompressed and installed one Qt component (Qt WebEngine) is bigger than the entire Sigil AppImage footprint.