iMovie increasing the size of video

I have a video which is about 100MB in size. I wanted to trim few parts in it, so imported the clip in iMovie project and trimmed it. Now when I export it to file, I get the video size of approximately 16GB (default). I understand that there are some export settings. Even when I chose Quality as Custom and reduce the slider to 2MBPS, the size still is about 500MB.

Why is iMovie increasing the size to 500MB even for the lowest quality when the original video size was 100MB and all I did was trim some parts and did no other editing. That just doesn't seem right. I would expect the video size to be lesser than 100MB since I only trimmed it.

I browsed through lot of forums and topics but couldn't find a proper answer for this. Can anyone help out?

Thanks in advance.