[FFmpeg-user] File size doubles with MKV to MP4 re-encoding?

[Ana B.]

Hello! I am trying to* re-encode MKV files into MP4 *and am getting pretty
desperate. FFMPEG changes my files to MP4 format, with the same quality
intact, but *the file size nearly doubles* with every conversion. (I don't
know if this is helpful, but using conversion programs like HandBrake
yields the same results: same quality but double the size of the original
MKV.) This is taking up a lot of GB on my laptop, but I absolutely need the
MP4 container to play my videos in Sony Vegas.

Here is the code that I run:

ffmpeg -i input.mkv -c:v libx264 output.mp4

Why is the file size doubling? Is there anything I can do to prevent this
or do you have any other suggestions? I seriously appreciate any help, *thank
you so much*!!