Newbie Here needing a little info

right i have seen that some tv files are like between 600 to 700mg and then a
couple of days later the same show is down here but like 350m hows this.. y is
this... and is this easy to do my self..

as i have a fair few files that size and would like to make them smaller
whilst keeping the sound intaked...

any help much thankfull

the 700 ones are hr hdtv which is highly picture quality compared to normal
hdtv (350).

to compress them, use autogk and set the target size to 350mb

cheers