Help with image quality please?

Hello everyone. I haave some 720p mkv files that I used Xillisoft Video Converter ultimate to convert to mp4 files (at highest audio and video resolutions) which then I put on a USB memory stick to watch on my 1080p television. The problem I am having is that when displayed on my TV, the image quality does not look HD, especially in very dark scenes, where it is very pixelated. I even tried reconverting to the HD mp4 setting to no avail. Would anyone have any ideas I could do to fix this? Thanks!