[JSWolf]

The problem is that almost every HTML file is too large are too large except for the cover. That's 45 HTML files that are too large. Some are way too large. They need to be split into smaller files. Use the calibre error checker in the editor to see which files are too large and to see when they are no longer too large after you split them. I don't know if Sigil's error checker checks if the HTML files are too large or not.

After the cover, the ToC is 1.1MB. The next file is 931.3KB Followed by 458.8KB, 479.5KB, and 1.8MB. Every HTML file except the cover is too large.