convertion vidio formats seem to give much larger file size than original?

hello all

I have a video file recorded from satellite which is good quality is a trp file and although a hd channel suspect video file is not actually hd codec etc.

the file will play via mem stick sat receiver to tv or similar files on usb hdd .

the problem is I have tried to convert this file to get similar quality/screen resolution and file size.

but every convertion format vob avi divx hd ps3 I have tried gives a far larger file size than the original.

using media info the original file is as follows.

file size 271mb 1920 * 1080 25fps avc high mpeg -ts

if I try to convert the smallest file size in say avi is about 500mb with a smaller screen resolution.

this has got me really baffled.

I do not know what avc high is?

thank you:)