Why is a screencast produced by Quicktime (.mov) is much smaller than the converted MP4 file? I recently recorded numerous screencasts using Quicktime.  The .mov files produced by Quicktime are 2876x1748.  

To goal was to upload the files to YouTube and so I wanted to convert to the recommended format.  Since I had to merge 3 clips into one, I used a tool called iSkySoft iMedia Converter Deluxe to merge and convert. 

The settings selected were optimised for YouTube (Settings were H.264 and resulting video was scaled down to 720P) 

Interestingly, the MP4 files produced were significantly larger than the original .mov files!  The .mov files combined were ~ 1.5GB at a higher resolution and the merged &amp; converted MP4 file was 2.2GB.

Is this to be expected?  Or could there be something wrong with the conversion tool being used?
I don't think it's the framerate. Maybe check with mediainfo to see more specific settings of each Codec. Apart from the bitrate, there are plenty of settings that can influence file size, even when using the same Codec.