What to do to leesen my video size?

MP4 files are already compressed to the optimum extent. The only way you can further reduce the size is by using a good video converter software to re-save them in a smaller resolution. Depending on the device you are watching them on, you will probably notice a significant loss of quality. The only other alternative is to buy more storage (such as an external hard drive) or to save them to DVD, either converted to video disk format, so that they play on a DVD player or to simply save the MP4 files as data files onto disc. You would then, however, probably need to copy the MP4 back to the hard disk when you wanted to watch the film.

Find a Video Convertor and many today come with compression features and even the format conversion. You can try converting to say another format keeping the quality almost similar.