CS5.5 - Frame changes after rendering

hi. i'm new here , have problem.

i started sequence videos(.mp4) filmed using gopro. 1920x1080 , 24fps

i created "new sequence clip". when drag video timeline, render bar yellow , video frame perfect

after render entire work area, render bar turns green frame becomes smaller , image quality worst.

why happening , fix this?

thanks help

what video previews settings sequence?

 

(go sequence>sequence settings...)

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