mp4 HD files converting

Got lots of films in mp4 720p but wont play on my panasonic dvd usb.
I used xilosoft to convert to MKV 720p but the file size grew from 780meg to 6+ Gigs
Did try SD MKV but quality very blotchy so just want to convert a file from mp4HD to MKV HD without the size growing to much.
Pal here in uk
Any ideas
Thanks
Answer #1
remux with mkvmerge …………. takes seconds