2 Videos with same resolution, 1 playes fullscreen, other with black bars

[jodiac]

My HDTV is a 40" Sony KLV-40R452A, I play most of my movies through the TV's USB port with 1TB portable HDD plugged in.
Some 1080p videos plays full-screen and some with black bars top & bottom. Same case for 720p videos also.

At one point I thought I figured out the 2 optimal resolution for 720p & 1080p videos to play full-screen . But my theory's gone to dust when I see that Two different Videos with same resolution doesn't play in the same way, 1 playes fullscreen, other with black bars.
Without any conversion, becasue my TV can play any format through USB, what can I do to play all 720p & 1080p videos in full-screen.

[Baldrick]

Many movies are wider than your tv(16:9) so they will have borders at top and bottom.

Just zoom on your tv or get a ultra widescreen tv, http://en.wikipedia.org/wiki/Philips_Cinema_21:9_TV .

[jagabo]

Two different Videos with same resolution doesn't play in the same way, 1 playes fullscreen, other with black bars.

Video files can tell the player to play any frame size at any aspect ratio. For example, NTSC DVDs use a 720x480 frame size for both 4:3 and 16:9 DVDs. The only difference is the instruction that tells the DVD player which aspect ratio to display.

what can I do to play all 720p & 1080p videos in full-screen.

Why would you want to? A 2.35:1 movie shouldn't fill your 16:9 screen. Neither should a 1.37:1 movie.