Compressing of DSLR video
Theoretically
Current DSLRs use H.264 compression for their video. Optimizing an already compressed image without losing quality is something commonly done for GIF, PNG or JPEG still images, and while it's theoretically possible to optimise compression in existing video files, currently no one bothered to develop the software to do so. The only solution left for you is to recompress your video with your codec of choice.
Don't try lossless codecs
While you won't lose any quality, lossless is only useful if your source material has a very high datarate. You will probably not gain anything compared to your DLSR files. (Your files seem to be about 25Mbit/s. Lossless x265/x264 will give you at least 100-200Mbit/s)
Try almost-lossless
Still, it's good to know that the internal (hardware) codecs of DSLRs are inferior to what your current CPU can do with recent software. But you should also know that any lossy recompression will result in an image that is slightly worse than the original, even if you encode in a higher quality than the original. That being said, here is a list of modern codecs that can compress your files with minimal loss: x264, x265, vp9 and av1 (to be released in 2017).
x264/x265 Settings for recompression with minimal loss

Encoding mode: constant quality. It will encode in one pass and output the lowest file size for a given quality setting. Your new file will contain variable bitrate video — your original DSLRs files are probably constant bitrate files
CRF quality setting: Choose a value between 12 and 20 (quality difference will be between indistinguishable and fine-for-the-less-discerning), if the source material is compressed in low quality, you can sometimes go higher than a CRF of 20 without noticing any difference. Incrementing CRF by +6 means about doubling the file size.
Codec: Compared to x264, x265 compression produces about half the datarate at the same CRF, but it is a lot slower.
Speed/Compression ratio: Use the veryslow setting to get the smallest file. Warning: this might be very slow ;-)

Depending on the above choices and on the complexity of the image and on the efficiency of the codec of your DSLR, your recompressed file will be up to 10 times smaller (probably less).
Handbrake is a nice app that will help you achieve all this. If you prefer the command line you should definitely go with ffmpeg:
ffmpeg -i DSLR.MOV -c:v libx265 -preset veryslow -crf 18 -c:a copy recompress.mp4