This is the file size. For a 720p file, 300MB suggests a "highly compressed" version (often using x265/HEVC encoding) to save data while maintaining decent quality.
If you are looking at a file named gaachi-part-1-2022-s01-hindi-ullu-originals-www-7starhd-social-720p-hdrip-300mb-1.mkv , here is what those tags mean: This is the file size
The file container (Matroska Video), which supports multiple audio tracks and subtitles. This is the file size
Some Ullu content is occasionally syndicated to other legal platforms like MX Player in certain regions. This is the file size