OK, I just did some experimenting.
I was able to get Sigil to go from about ~200MB of Memory use, up to >50 GB of memory use, in about 30 seconds.
All I had to do was grab the boundary for the "Preview Window" and resize it about 20 times, through click-and-drag.
Obviously, this would have maxed out my 4090, and then spilled over into my RAM to get this high.
Whose responsibility is this? Can we make a bug report and have it patched by the same people who fixed the Mac version, as already indicated?