mikokofiles "original compressed with same quality" - That's not really possible.
I don't have a paid account so my capacity is limited. I've downloaded a few of the files to see what kind of compression is possible. Here are the details from my little experiment on 7 files chosen at random.
0001 06m 09s (1920x1080) 60MB -> (1280x720) 44MB
0002 05m 53s (1920x1080) 57MB -> (1280x720) 37MB
0066 32m 48s (1920x1080) 887MB -> (1280x720) 63MB
0075 29m 31s (1280x720) 140MB -> (1280x720) 168MB [20% larger file]
0082 36m 50s (1280x720) 210MB -> (1280x720) 249MB [19% larger file]
0085 40m 18s (1280x720) 326MB -> (1280x720) 262MB [20% smaller file]
0133 14m 00s (1920x1080) 1082MB -> (1280x720) 123MB
As you can see there are some savings when reducing the resolution from 1920x1080 to 1280x720. There is a significant saving if the original video wasn't compressed very well as was the case for 0066 and 0133. However there seems little point in trying to reduce the size of the videos which are already at the 1280x720 resolution, from my limited experiment using FFMPEG the files are more likely to increase in size.