My output file is larger than my input. What's going on?
Occasionally an output file created by Zencoder will end up being larger than the input file. This typically happens when the input file is highly compressed or contains visual artifacts.
During transcoding Zencoder can't tell the difference between the actual content of the video and artifacts caused by compression, so it tries to preserve the artifacts in the video. Deliberately including this in the output takes more space compared to accidentally having it due to compression, which causes the outputs to be larger in size.
If your input files are compressed and you want to keep file sizes down your best bet is to determine the bitrate of the input file before sending it to Zencoder and setting your output's bitrate to match it. Otherwise, it is preferrable to send less compressed videos that contain fewer artifacts in them. This will result in better looking outputs that use less disk space.
Additionally, some codecs are less efficient than others. VP6, Theora, and MPEG-4, for example, are generally less efficient than H.264 and VP8. They're still fine codecs, but converting from a highly efficient codec to a lower one will obviously impact the file size.