Merging flv files causes a massive size increase

I have 3 flv files which I wish to merge. I used the answer in this post: https://unix.stackexchange.com/questions/45870/merge-multiple-flv-files which suggested this option: mencoder -forceidx -ovc copy -oac pcm -o output.flv input1.flv input2.flv It seems to have worked except that the merged file is 400meg in size whereas the original 3 files were about 33meg each. Is this expected? Is there a way to join the 3 files without the massive increase in size?