Compression is dependent on what you are compressing as well as how much RAM you have. Pre-compressed files like pictures and videos are more difficult to compress than files like text, or directories where there is a lot of very similar or same files.
I have six backup directories of a personal php website (xampp) with some mostly minor differences in them. Each main directory is about 600M to 1Gig give or take. Totaling 6gig for all directories together, again, all holding similar files.
Dictionary size to compressed size for all directories in a single package.
4M dictionary = 1,687,995KB
24M dictionary = 1,685,337KB
128M dictionary = 1,685,336KB
512M dictionary = 1,685,336KB (no change from 128)
1024M dictionary = 315,224KB
since every directory is roughly the same, and are larger than 512M the 1024M dictionary seems to be the best in this situation.
Compressing 6gigs worth of folders of different music using the same 1024M dictionary size resulted in only 96% ratio of compression. 5.76 gigs instead of 6gigs.The best thing to do for compression of video is lossy compression where you use a program to convert the video. Try lowering the bit-rate to something you don't notice, or can accept the video quality of. Handbrake is a decent video tool, but there are many. VLC is capable of video compression as well using the convert option. Both programs are free to use.