10gb for a single tv episode is huge. Incredibly big in fact.
An .m2ts file can have any one of several different video codecs within it. Most of the time they use H264, but can be VC1 or mpeg2. Mpeg2 is the least efficient in size, so this one may have mpeg2 in it now.
You can find out by examining the file with MediaInfo (a free program). http://mediainfo.sourceforge.net/en
Have you tried playing the m2ts file with VLC off the hard drive? It should be able to do this without conversion. You can also try Media Player Classic Home Cinema. http://mpc-hc.sourceforge.net/
In order to fit a 10gb file onto a 4.3gb single layer dvd, you'd need to apply a lot of compression. If the file is already H264 or VC1, converting to mpeg2 would make it larger, if you keep the same quality in the encoding. Converting to mpeg2 and compressing to fit that single layer dvd target may require losing HD resolution...in other words, you might need to make a regular dvd-video out of it. MultiAVCHD should be able to do this. ClownBD has recently added the ability to produce dvd-video from blu ray m2ts files, so that might be an option too. A third choice would be AVStoDVD if you want to make a dvd-video from the m2ts file.
You could also make an AVCHD disk and keep the HD resolution, but the video would be encoded with H264 and would only play on a blu ray player with AVCHD support or in your computer.