
I think you misunderstood the concept of the ProRes Codec.
Your Original Media like m2ts-files are (usually) always the smallest files because they are compressed the highest.
Now:
Because the original Media like for example mp4 are compressed so high they are hard to edit because they need a lot of processing power from your computer.
For example scrubbing over a 1080p50 m2ts clip is quite choppy even on maxed out computers.
This is where the ProRes Codec comes into play.
The ProRes Codec does not compress the video very high so the computer doesn't need so much processing power to decode these files. So scrubbing over clips works like a charm for ProRes Media. However those ProRes files are quite bigger than the original media but are better useable for editing in FCPX.
There are two ProRes variations:
Proxy or Optimized
Optimized Media is are very big files.
Proxy Media files are not as big as optimized files but still bigger than the Original Media files.
Optimized Media is looking almost as good as the original files.
Proxy Media looks not so good.
When rendering your final video the original media is use by FCPX for maximum quality
Remember: Proxy and Optimized are not used to get smaller file sizes. Their purpose is to offer comfortable editing.