My understanding is that Shotcut doesn’t generate an optimized palette for GIF files. That means any exports will probably be the same size and appearance regardless of export settings.
GIF isn’t like other formats where bigger automatically means better. GIF is an indexed image format into a 256-color palette, meaning frames are pretty much always the same size (barring basic compression) unless a custom palette is used for every frame. But that isn’t a recommended option for compatibility and many other reasons.
For high-quality GIF conversion, consider exporting the Shotcut video as Ut Video Lossless, then use FFmpeg to generate an optimized palette. The command looks like this:
ffmpeg -i Input.mp4 -filter_complex "[0:v] split [a][b];[a] palettegen [p];[b][p] paletteuse" Output.gif
More details here: