And yet you appear to have made the exact same mistake again in the very next sentence! I would be wise to get the correct frame rates in your mind otherwise it's likely to bite you in the a$$ one day!
Presumably you mean 29.97fps or 30fps and 23.976fps again?
There are a couple of possibilities: The most likely is that most programs are not very accurate when it comes to estimating file size, especially with lossy codecs. It could be that if you were to render them out the actual file sizes would be as one would expect (IE. Roughly 1/5th smaller for the 23.976fps rather than 1/2). A less likely possibility is that as 1080p 30fps is not a standard frame rate/resolution, it might cause different algorithms to be employed or might even be changing some rez settings, either of which could potentially make a bigger difference than the frame rate difference alone would indicate.
Depends on a number of variables; what codecs one is using to transcode, what setting are employed, etc.
You do realise it's a Command Line program? It does not have a gui, clicking the program will not do anything, you need to run it from a command line prompt.
G