Smaller frame, lower frame rate
The Email recipe writes a 960×540 (16:9) MP4 with H.264 video and AAC audio at 128 kbps. The frame rate follows your video up to 30 fps, so a 60 fps clip loses half its frames. A 4K or 1080p original comes down to 960×540.
A vertical phone video is fitted into the wide frame as a narrow column, with a blurred copy of itself on both sides. If you want to keep the vertical shape and simply make the file lighter, ask ToolGPT to compress it instead of using the recipe.