You can also purchase or rent the movie through digital stores like Google Play Movies, iTunes, or Amazon Video. This way, you're supporting the creators and getting a legal copy.
While downloading from the above sources is common, it often involves pirated content. To watch 300 safely and legally, you can find it on most major Over-The-Top (OTT) and digital rental platforms such as: Download 300 -2006- Dual Audio -Hindi-English- ...
A real 720p movie will never be only 20 MB. If the file size looks unnaturally small, it is likely malware. You can also purchase or rent the movie