I used Movie Maker 12 to trim the first 30 seconds from a 45 minute MP4 movie. The original and edited movie are HD 1080. The original file was 800 Mb and the edited file is 2.7 Gb. The file size always increases after edits of MP4, WMV, and FLV files when saving in the same resolution as the original. Why does the file size increase? and how can I prevent this happening?

For information, I have used Avidemux to perform the same operation and the file size becomes slightly smaller which is good but it isn't as accurate as Movie Maker when selecting the actual start and end points of the trim.

Any help on this subject will be very  welcome.

Best regards