Impeller renderer is about 100 KB [0]. You still need to add things like text layouting library and image codecs to that size. A few MB in total I guess, but much less than a full web engine.
While Flutter could be used to draw UI to texture, Impeller could also be used by other UI frameworks. Recently, Avalonia team experimented with replacing Skia and a Flutter developer asked, if they are interested in using Impeller and even offered some help [1].
This would probably be suitable, as a more lightweight alternative to WebView, for integrating into game engines.
Even if you don't opt for gpu compositing, you can be 8x more performant than skia by writing something yourself. Skia is optimized for portability, not necessarily performance.
While Flutter could be used to draw UI to texture, Impeller could also be used by other UI frameworks. Recently, Avalonia team experimented with replacing Skia and a Flutter developer asked, if they are interested in using Impeller and even offered some help [1].
This would probably be suitable, as a more lightweight alternative to WebView, for integrating into game engines.
[0] https://chromium.googlesource.com/external/github.com/flutte...
[1] https://www.reddit.com/r/dotnet/comments/1nv3snm/comment/nh7...