3D blurays aren't double the size when you remux them.
The right-eye data is never the same size as the left-eye.
The 3D depth is never so severe that it amounts to the same size.
For the most part, it's just very light 3D with some heavy 3D scenes so that's why the size can be anywhere from 1/3 of the left-eye to 1/2.
You can tell how much effort was put into the 3D production of the movie based on the remuxing.
In most cases, it's all post-processed so there's really nothing there.
Naturally filmed 3D with 3D cameras should have more difference when remuxed than some cheap after-thought movie.
If you used MakeMKV and selected the MVC stream and checked in MediaInfo and it says "Stereo", then it will be there.
The part that's throwing you off is not knowing how to properly
view the MVC stream.
Playing it in a player, the player needs to be a 3D player (there are a lot out there that are limited; this requires a lot of research) and the player needs to be hooked up to a 3D monitor.
With MPC-HC + LAV (no ffdshow) + MadVR, you can view it in SBS. Or whatever you set it to. Then it's up to the monitor to play it in 3D.
You can start here:
http://forum.doom9.org/showthread.php?t=170828
It links to other tools like:
DGMVCSource (It's under a new domain:
http://rationalqm.us/mine.html)
FRIMSource
I've used DGMVSource to view the right-eye stream, in the past.
For DGMVsource, it's been a long time since I tried it, but IIRC, it doesn't work well with random access.
It's better if you convert the MVC stream to something like a lossless intermediate (UT Video Codec, etc).
For 1080p content, this will take up a lot of space but you'll have better random access for jumping to different points of the movie.
These require Avisynth usage. All of it, except for using FFMPEG directly.
For FFMPEG, I believe if you force FPS to be 48 (or double, 23.976*2,25*2,etc), it should give you both frames, alternating.
Frame 1 2D.png
Frame 1 3D.png
Frame 2 2D.png
Frame 2 3D.png
But it won't actually label which frame is 2D and which comes from 3D. You just have to assume that the left-eye comes first, so every odd frame will be the 2D data and every even frame will be the 3D data.
With 3D DCP trailers in .mxf containers, I've converted to lossless intermediate and forced 48fps and it would do that for me.
Opening the lossless.avi in Avisnyth, I would either SelectOdd() for 2D or SelectEven() for 3D.
Playing sequentially, you'd have a seizure.
Never really tested that method for avc/mvc stereo files.