Unlike what Chas_m said, The Macbook screen is 1280x800, which is a 16:10 resolution (not 16:9 - the closest to actual 16:9 that screens get at about that resolution (1280x720) is 1366x768). Any movie that has its actual dimensions at 16:10 will be full screen with no "black bars", any movie that is 16:9 or any resolution other then 16:10 will have "black bars". Just like if you play some movies on a 1080p TV, if the movie is 1920x960 (or some other proportion, which many are) they will show black lines above and below because there just isn't data there - the only way to get rid of them is to have it scale the picture to fit the height component which is a terrible idea because then you loose left and right content.
Any 1080p movie will be scaled to fit on the screen of the laptop (or the play window if not playing full screen) - there is just no choice on this (whether the movie is 1920x1080 (or 1920xsomesmallernumber) or 1440x1080 - it just won't fit on a 1280x800 screen without being scaled).
720p movies, if at 16:9 will have black bars, at max rez, 720p is 1280x720 - this will leave 80 unused pixels on the screen, 40 above and 40 below the image (assuming a max frame size 720p video).
As Chas_m stated, the Macbook Pro is more then capable of playing 1080p video. Depending on the encoded bitrate, you may need to use a video player that is capable of using hardware acceleration for playback (ie: if your video files are .mkv files, quicktime won't play them natively and won't necessarily apply hardware acceleration so you'll want to play those back through something else, like Plex or Movist, etc.) I had an earlier 13" MBP and frequently played HD footage on it (720p and 1080p) including having it connected to HDTVs for video playback.
