The weight of the video is a consequence of its quality (image resolution, number of frames per second, soundtrack quality, etc.) and its duration.

On average, a 1-minute video exported in .mp4 format in HD (1920 pixels wide by 1080 pixels high, i.e. "1920*1080", i.e. "1080p") weighs 70 MB.

In addition, as long as you keep the movie project related to this exported movie (for possible editing), then a copy of all sequences and all setting parameters of this movie are retained, approximately 70 MB more.