hmm file gets to big but why?

[kimpan]

I am encoding a 100 min movie with tmpgenc. I used sorce range and I ´set the start frame in the middle of the movie and the end frame in the end. But when I start encoding I get a black screen, not untill i have done 50% of the encoding I will see the picture. It starts in the middle just like I told it to at start frame. But the filesize is the double of what it should be. I see this: MPEG-1 352x288 25fps VBR 1550kbps, Layer-2 44100Hz 224kbps

it get to 1400mb. what to do??